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MATH137 - September 9-December 2, 2013

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Mathematics
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MATH 137
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Sean Speziale

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⇚ Return to University Notes index MATH 137 Calculus I: Section 003 Instructor: Name: Sean Speziale Email: [email protected] Office: M3 2128 Advising Hours: Monday, Wednesday at 12:00-13:30, MC 4023 Office hours: starts week 3, details TBA ;wip 9/9/13 Preliminaries IntegersZ = …,−2,−1,0,1,2,… p Rational numbersQ = q,p ∈ Z,q ∈ Z Irrational numbers: cannot be expressed as ratio, non-terminating and non-repeating decimalπe,e,√s2,ln(2). Real number (denotedR): containZ,Q , and irrationals; any number with decimal expansion as opposed to complex/imaginary numbers. Complex number (denotedC ): contains real and imaginary parts. Intervals/Sets [a,b]- inclusive (closed) intervaa tob. (a,b]- left exclusive (left open) interato b, not includan. [a,b)- right exclusive (right open) interaatob, not includib. x ∈ (a,b) -x is an element (a,b)and {x ∈ R a < x < b} . Infinity can be used as an interval endpoint, but is always excluded as it is a l[4,∞) n.t a number: Inequalities ▯ - if and only if; first implies second, and second implies first. Solve6 < 1 − 3x ≤ 10 : 6 < 1 − 3x ≤ 10 Isolate x: 5 < −3x ≤ 9 −5 > 3x ≥ −9 −5/3 > x ≥ −3 Solve 1 < 2 : x 1 We know that x< 2 . 1 Ifx > 0 ,1 < 2x , sox > 2. Ifx < 0 ,1 > 2x , sox < 1. 1 2 1 Ifx > 0 and x > 2 , thex > 2 . Ifx < 0 and x < 1 , thex < 0 . 2 Therefore,x ∈ (−∞,0)∪ (1/2,∞) . Absolute Value Distance between number and 0 on the real number line. x, if x >= 0 x = { −x, if x < 0 |x − a| is the distance betwexnand a on the real number line: x − a, if x >= 0 |x = { a − x, if x < 0 Solve by considering both cases. Deal with each case separately. Solve|2x − 3 < 4 : Open the inequality: − 4 < 2x − 3 < 4 |f < g ▯ −g < f < g −1 < 2x < 7 ▯ − 1 < x < 7 2 2 Simplify|x + y ≤ 1 : y ≤ 1 − x | 1 − x if x ≥ 0 y ≤ { 1 + x if x < 0 11/9/13 Functions A function is a rule that assigns a single output to an input. x (independent variable) -> f -> f(x) or y (dependent variable) A function maps values in the set known as a domain into values in the set known as a range. Domain: set of allowable values for the independent variable. Common exclusions from domains: Dividing by 0. Logarithm of non-positive number. Even root of negative number. A function has an inverse if and only if every unique value of the independent variable in the domain has a unique value of the dependent variable - i.e., "one-to-one". Range: set of possible values for the dependent variable. Finding the range can be significantly more difficult than finding the domain, which is simply a matter of looking at the function. f(x) = √x − 1− Restriction: x > 1 D = [1,∞) R = [0,∞) 1 f(x) = 1 − x2 Restriction: x ≠ 0 D = (∞,−1)∪ (−1,1)∪ (1,∞) R = (−∞,0)∪ [1,∞) A true function must have a unique output for each input. Graphically, the vertical line test passes for all points (vertical line drawn on graph will only intersect it at most once if it is a graph of a true function). Non-functions can be described using functions by combining multiple functions into one: −2−−−−2 Unit circy = ± √ x + y −−−−−− −−−−−− This can be written as two funcy = −:√ x + y 2 and y = √ x + y 2. Horizontal line test: a function is one-to-one if any horizontal line drawn on its graph intersects it at most once. Function examples: n Polynomialx , n is a positive integer (domain real numbers) Rationalf(x)/g(x) (domain real numbers such thg(x) ≠ 0 ) Trigonometric/Inverse trigonometric Exponentialxn,x is a positive real number (domain real numbers) Logarithm:log(x)(domain positive real numbers) 13/9/13 Transformations Sketchy = x 2,y = x − 1 y = (x + 1) 2: 2 y = x 2is simply a parabola. y = x − 1 is the same parabola moved down 1 unit. y = (x + 1)2 is the same parabola moved left one unit. Given functiof(x): f(x)+ c is a vertical transcaunits upward. f(x + c) is a horizontal translcunits to the left. cf(x) is a vertical stretch by a fc.tor of f(cx) is a horizontal compression by a facc.r of −f(x) is a reflection along the x-axis. f(−x) is a reflection along the y-axis. |f(x)|is a reflection of all parts of the function below the x-axis along the x-axis. f( x )is a reflection of all parts of the function to the right of the y-axis along the y-axis replacing what was on the left. Compositions Given two functionsf(x) and g(x) : f ∘ g (equivalent tf(g(x)) ) is a composition of and g. The domain of f ∘ g is all values xfin the domain ofg for whichg(x) is in the domain of . 1 −− −−− Given f(x) = x,g(x) = √ 1 − x , finf ∘ g: f ∘ g is equivalent to 1 . √1−x Restriction on square root1 − x ≥ 0 , sx ≤ 1 . Restriction on divisiog(x) ≠ 0 , sx ≠ 1 . Domain of f ∘ g:(−∞,1]∪ ((−∞,1)∪ (1,∞)) is equal t(−∞,1) . 16/9/13 Symmetry A function is even if(−x) = f(x) - the function is symmetric about the y-axis. 2 Examples include y = x | and y = x . A function is odd if(−x) = −f(x) . Examples include y = x and y = sin(x) . One-to-one Functions A function is known as one-to-one on an interval if it never takes the same value twice - it passes the horizontal line test. Methematically, for any two numbers x1∈ I,x ∈2I f(x ) ≠1f(x ) 2 whenever x 1 x 2. Graphically, when we take the inverse of a function, we reflect it along y = xin. So the horizontal line test is actually the vertical line test of the inverse. A function is monotonically increasing on an intervalI if for anx1∈ I,x ∈2I x < x1▯ 2 f(x 1 < f(x )1 . Likewise, a function is monotonically decreasing on an intervaI if for anx ∈ I,x ∈ I , 1 2 x1< x ▯2 f(x 1 > f(x )1 . A function that is monotonically increasing or monotonically decreasing on an interval is always one-to-one oon that interval. Show that f(x) = x 2is not one-to-one overR but is ove[0,∞) : Counterexample: Since, f(−1) = f(1) , the function is not one-to-one. We can prove the latter statement by proving that the function is increasing over the interval. Let x ∈ [0,∞) and x ∈ [0,∞) represent two arbitrary values such thx < x . 1 2 1 2 Then f(x2)− f(x )1= x − 2 = (x1− x )(2 + x 1 1 2 . Both terms are always positive, sf(x )− f(x ) > 0 and f(x ) > f(x ) . 2 1 2 1 Therefore, the function is monotonically increasing. Inverse A function has an inverse if and only if it is one-to-one. By definition: Letf(x) be a one-to-one function with domainA and range B . The inverse function of(x) , denotedf −1(x) , is defined fy−1 (y) = x ▯ f(x) = y . The range of the inverse is the domain of the function, and the domain of the inverse is the range of the function. An inverse of a function "undoes" the operation applied by the function. In other words, the cancellation equations −1 −1 ∀x ∈ A, (f(x)) = x ∀x ∈ B,f( (x)) = x ∀x ∈ A,f −1(f(x)) = x and ∀x ∈ B,f(f −1 (x)) = x hold for all functions with inverses. −1 −1 Note thatf (x) is not the same f(x) - the former is the inverse and the latter is the reciprocal. The inverse is found by swapping the variables and isolating variables. If a function is not one-to-onR , we may restrict the domain to a region where it is, and define the inverse there. This is useful with things like trigonometric functions, for example. −−−−−−− Find the inversef(x) = √ 10 − 3x : 10 D = (−∞, 3 ] R = [0,−−−−−−−−−−− −1 x = √ 10 − 3f (x) 2 −1 x = 10 − 3f (x) x − 10 = −3f −1 (x) 2 10 − x −1 3 = f (x) D = [0,∞) 10 R = (−∞, 3 ] Graphically, if the (a,b) is on the graph f(x), the(b,a) is on the graph ff1(x) . Using this information, we can sketch the inverse of a graph by reflecting it aloy = xe.line 18/9/13 The Constant e e is a constant used in many places in mathematics. It is approximately equal to 2.718. There are several ways to dee:ne 1 n As a limie = x→∞ (1 + x ) . As an infinite sere =:∑ ∞ 1. n=0 n! x Graphically with tangents: of all exponential equations oy = a ,a > 0 , only whera = e does the tangent of the graph have a slope ofx = 0 . a − 1 Also,a = e is the only value for wlimh = 1 . h→0 h Exponentials/Logarithms The functiof(x) = e xis called the natural exponential functionf(x) = ln(x) is called the natural logarithm. x Sketchf(x) = 3 + 2e : Start with graph f(x) = ex. Reflect the graph along the Y axis. Stretch graph vertically by a factor of 2. Move the graph up by 3 units. The functiof(x) = a x is either increasina > 1 or decreasing f0 < a < 1 , and therefore has an inverse. This y inverse is calloga(x).log ax) = y ▯ a = x . x The domain off(x) = a isR , and the range{y ∈ R x > 0} . Therefore, the domainf(x) = loga(x) is {y ∈ R x > 0} and the range Rs. ln(e) = log (x) e ln(e) = 1 ln(e) = 1 1 loga(x ) = rlog (a ) r . Solog(√x) = 2 log(x) . 2x+1 Find the inverse of(x) = e : −1 x = e 2f (x)+1 −1 ln(x) = 2f (x)+ 1 1 1 f −1(x) = ln(x)− 2 2 20/9/13 Trignomonetric Functions a Angles are measured in radians. Radians are the ratio of the associated arc and the θ =iur:. For a full circa = 2πr , sθ = 2πr = 2π . r Radians Degrees π 180 π 2 90 π 3 60 π 45 4 π 6 30 These are based on the special triangles: π π π Right triangle with side lengt1,1, 2 and internal angles4 ,4 . 2. Right triangle with side lengt1, 3,2 and internal anglesπ ,π .π . 6 3 2 2 2 Parametric equations of unit circx + y = 1 ):x = cosθ and y = sinθ together form a unit circle from polar coordinates. 2 2 x + y = 1 sin θ + cos θ = 1 Properties Bounds: −1 ≤ sinθ ≤ 1 ,−1 ≤ cosθ ≤ 1 Periodicitysin(θ + 2πk) = sinθ,k ∈ Z ,cos(θ + 2πk) = cosθ,k ∈ Z Odd/even: sin(−θ) = −sin(θ) - odd functioncos(−θ) = cos(θ) - even function Sign:cos is positive to the right of the ysiniis positive above the x-axis Other trigoometric functions: Tangent: tanθ = sinθ c1sθ Cosecant cscθ = sinθ Secant: secθ = 1 cosθ Cotangent: cotθ = cosθ sinθ Identities 2 2 θ + θ = 1 Square Identisin θ + cos θ = . Addition formulsin(x + y) = sinxcosy+ cosxsiny ,cos(x + y) = cosxcosy− sinxsiny . Find the subtraction formulas: sin(x − y) = sinxcos(−y)+ cosxsin(−y) = sinxcosy− cosxsiny cos(x − y) = cosxcos(−y)− sinxsin(−y) = cosxcosy+ sinxsiny Inverse Trigonometric Functions Trignometric functions are periodic. Therefore, they are not one-to-one on the real number line and cannot have an inverse on this interval. However, they are one-to-one if we restrict the domain. Conven[−ona, ]for sine a[0,π]for cosine. 2 2 −1 The inverse of the sine function x = arcsinθ, ox = sin θ - "the angle whose sxnin the restricted domain". Its domai[−1,1]and its range[−s2, 2. The inverse of the cosine function arccosθ, ocos−1θ - "the angle whose cosine is x in the restricted domain". Its domain i[−1,1] and its rang[0,π]. The inverse of the tangent function arctanθt, otan−1θ - "the angle whose tangent is x in the restricted domain". Its domaiR and its rang(−isπ, ) . 2 2 Evaluatcos(arctan√ 3): √3 opposite = 1 adjacent opposite = √3 adjacent = 1 −−−−−−−−−−−−−−−−−− hypotenuse = √ opposite + adjacent2 adjacent 1 cos(arctan√3) = = −−−−−−−−−−−−−−−−−− hypotenuse √ opposite + adjacent2 1 1 cos(arctan√3) = −−−−− = 2 √3 + 1 Simplifsin(arctanx) x opposite 1 = adjacent opposite = x adjacent = 1 −−−−−−−−−−−−−−−−−− hypotenuse = √ opposite + adjacent2 sin(arctanx) = opposite = x hypotenuse −−−−−−−−2−−−−−−−−− 2 √ opposite + adjacent x sin(arctanx) = −−−−−− √ x + 1 ( ( π)) Evaluatarccos sin − 3 : pi/6 |\ | \ 2 sqrt(3) | \ |___\ pi/2 1 pi/3 √ = = − −π opposite √ 3 sin ) = = − 3 hypotenuse 2 opposite = √3 hypotenuse = 2 −−−−−−−−−−2−−−−−−−−− 2 −−−−− adjacent =√ hypotenuse − opposite = √4 − 3 = 1 π π 5π arccos sin − 3 )) = π − 6 = 6 23/9/13 Hyperbolic Functions The hyperbolic functions are expressible as combinations of exponential functions. e −ex sinhx = 2 ("sinch ex") - hyperbolic sine function. DomR.n and range is e +ex coshx = x2−x("cosh ex") - hyperbolic cosine functRoand range [1,∞]. tanhx = e +ex= coshx hyperbolic tangent function.RDand range (−1,1). There are also hyperbolic secant, cosecant, and cotangent functions, but they are rarely used. e−−ex 1 x Hyperbolic sin is an odd sinh(−x) = 2 = −sinhx . Ax → ∞ ,sinhx approache2e asymptoticallyx → −∞ ,sinhxapproache− 1e−xasymptotically. ex+e Hyperbolic cosine is an even cosh(−x) = 2 = coshx . Ax → ∞ ,sinhxapproaches2ex 1 −x asymptoticallyx → −∞ ,sinhxapproaches2e −xsxmptotically. Hyperbolic tangent is an odd tanh(−x) = e−+ex = −tanhx . Ax → ∞ ,tanhx approaches 1 asymptoticallyx → −∞ ,tanhx approaches -1 asymptotically. 2 2 If we x = coshtand y = sinh, thex − y = 1 . This is a hyperbola. The graph appears as an hourglass shape. 25/9/13 Inverse Hyperbolic Functions Hyperbolic sine is one-to-Rn. over To find the inverse functions, yewherex = sinhy = e −e: 2 y −y x = sinhy = e − e 2 2x = e − e−y y −y 0 = e − 2x − e 0 = e e − e 2x − e ey y 2 y (e ) − 2xe − 1 = 0 −−−−−−−−−−−−−−− 2 y 2x ±√ (−2x) − 4 ⋅1 ⋅−1 −−2−−− e = = x ±√ x + 1 −−−−−− −−−−− 2 Since x −√ x + 1 < 0, e = x − √ x + 1 is extraneous. y −2−−− e = x + √ x + 1 √ −−2−−− y = ln(x + x + 1) With this technique, we discover the inverses of the hyperbolic functions are: −1 −−−−− x = ln(x +√ ) sinh−1x = ln(x + √ x + 1) −1 √ −−2−−− cosh−1x = ln1x +1+xx − 1),x ≥ 1 tanh x = 2ln(1−x),−1 < x < 1 Findtanh x1 : e − e −y x = tanhy = y −y e + e xe + xe −y = e − e −y xe + x = e 2y− 1 2y (x − 1)e = x + 1 x + 1 e = ± √ x − 1 x + 1 Since − √ < 0, the negative solution is extraneous. x − 1 y −x + 1 e = √ x − 1 x + 1 2 y = ln( ) x − 1 1 x + 1 y = ln 2 x − 1 Limits Limits are a core concept in calculus, because they appear in the definitions of derivatives and definite integrals, the two main branches of calculus. A limit does not depend on what the value of the function is at the point it is approaching. Consider the following function: g(x) = { 3x − 1 if x ≠ 1 0 if x = 1 When we say the limitf(x)isL asx → a , we meanf(x) gets closer and closLrasoxgets closer and closea.to The precise definition of a limit: Letfbe a function defined on an open interval that contains a, except possiblaitself. Then we say the limif(x) asx approachesa iL , if for every nuϵ > 0 there exists a numδ > 0 such tha0 < x − a < δ ▯ |f(x)− L < ϵ . We write thilimf(x) = L , of(x) → L as x → a. x→a In other word(∀ϵ > 0,∃δ > 0,0 < x − a < δ ▯ f(x)− L < ϵ) ▯ (lix→a f(x) = L) . ϵis Epsilon (Greek) and is associated with error. δ is Delta (Greek) and is associated with difference. The key point is that the limit exists if the dif(x)candeLw(the errϵ) can be made as small as needed by making the distance betwxeand a(the differenδ) sufficiently small. Hox ≠ a,. We can also write the implic0 < x − a < δ ▯ |f(x)− L < ϵ as a − δ < x < a + δ ▯ L − ϵ < f(x) < L + ϵ . δ depends onϵ. 27/9/13 Geometry and intuition are useful tools, but do not prove anything. Considerf(x) = sin(x) . The graph has higher and higher frequency oscillations as it approaches 0 from either direction, to a limit of infinity. Consider the following function: f(x) = { 3x − 1 if x ≠ 1 0 if x = 1 How close must x be to 1 to ensure that h(x) is within 0.1 of 2? 0.1 plays the role ofϵ. We need to find the corresponding value of δ > 0 such that 0 < x − 1 < δ ▯ h(x)− 2 < 0.1 . So |3x − 1 − 2 < 0.1 , or3 x − 1 < 0.1 . So |x − 1 < 0.1 , or1 . 3 30 To prove that a limit exists, we need to use an arbitrarϵ > 0 : We need to find the corresponding value of δ > 0 such that 0 < x − 1 < δ ▯ h(x)− 2 < ϵ . So |3x − 1 − 2 < ϵ , or3 x − 1 < ϵ . So |x − 1 < ϵ. 3 ϵ So if we choose δ = 3 , then we know that 0 < x − 1 < δ ▯ |h(x)− 2 < ϵ . Therefore, the limit exists, by definition. Heaviside Function 1 if x ≥ 0 H(x) = { 0 if x < 0 Prove that the limit does not exist ax = 0 : Proof by contradiction. Suppose the limit does exist. So ∀ϵ > 0,∃δ > 0,0 < x < δ ▯ | |H(x)− L < ϵ | . Consider ϵ = 0.2 . So 0 < x < δ ▯ L − 0.2 < H(x) < L + 0.2 . So the distance between L − 0.2 and L + 0.2 is 0.4. Let −δ < x <10 and 0 < x <2δ . So it must be that and . H(x )1= 0 H(x )2= 1 So the distance between the two is 1. But the distance between the two must be less than 0.4. Therefore, the limit cannot exist foϵ ≤ 1. 2 30/9/13 Limit Laws/Theorems For complicated functions it is often impractical to use the definition to prove limits. Instead, we use limit laws, which can be proved from the definition. Limit sum law Iflim x→a f(x) = L and lim x→a g(x) = M , thenlim x→a (f(x)+ g(x)) = L + M . Proof: Iflim x→a (f(x)+ g(x)) = L + M , then ∀ϵ > 0,∃δ > 0,0 < x − a < δ ▯ | |(f(x)+ g(x))− (L + M) < ϵ | . Clearly,0 < x − a < δ ▯ (f(x)+ g(x))− (L + M) < ϵ | is equivalent to 0 < x − a < δ ▯ (f(x)− L)+ (g(x)− M) < ϵ | . By the triangle inequality,(f(x)− L)+ (g(x)− M) ≤ f(x)− L + g(x)− M | | . ϵ We want to make each term less than 2 , so later we can add them together to get ϵ. Sinceϵ is arbitrary,> 0,∃δ > 0 0 , x − a < δ ▯ |f(x)− L 0,∃δ 2 0 0,< x − a < δ ▯ 2 |g(x)− M 0 Clearly, this is equivalent to . Since the left side limit does not equal the right side limit, the limit −1 if x < 0 does not exist. Approaching from either side gives different results. This leads to the idea of a one sided limit. lim x→a− f(x) is the left side limit - approacaifrom below/left. + lim x→a f(x) is the right side limit - approacaifrom above/right. limf(x) = L ▯ lim−f(x) = lim f(+) = L x→a x→a x→a The limit exists if and only if the limits on either side exist. 2/10/13 Vertical Asymptotes At vertical asymptotes, the limit approaches positive or negative infinity. Consider f(x) = 12 nearx = 0 . Asx → 0 ,f(x) grows without bound. x We say that f(x) → ∞ asx → a , orlimx→a f(x) = ∞ , if for eveM > 0 there existsδ > 0 such thatf(x) > M whenever 0 < x − a < δ . In other words, the value f(x) can become arbitrarily large for some value xfclose enough to a. Likewise, with a downwards asymptote likef(x) = − 1 near x = 0 , the same behavior applies, excepf(x) → −∞ as x2 x → a , olim x→a f(x) = −∞ . 12 6 Given f(x) = e x , determine how close must x be to 0 so thaf(x) > 10 : In other words, finδ > 0 such thatf(x) > 10 6 when 0 < x < δ . 6 1 1 2 1 Clearly,f(x) = 10 is equivalent tx2 = 6ln(10) , andx2 = 6ln(10) . Sox = 6 ln10 So |x < 1 . √6 ln10 In rational functions, vertical asymptotes usually occur when the denominator becomes 0. x +1 Find the left and right hand limitf(x) = 3x−2x2 at each asymptote: x +1 3 Clearly,f(x) = x(−2x+3). So the asymptotes are ax = 0 and x = 2 . − 2 As x → 0 ,x + 1 > 0 − ,x < 0 , and−2x + 3 > 0 . So f(x) < 0 forx = 0 . So f(x) → ;wip: use the sign analysis with infinismals Continuity We say f(x) is continuous aa iflim f(x) = f(a) . x→a If this is true foa ∈ I , withI being an interval, we sf(x) is continuous onI. This implies the following conditions must be met: f(a) must be defined -a must be in the domain off(x) . The left and right limits are equal. f(a) is equal to the limit at that location The most common discontinuities are: 1 Infinite discontinuity - vertical asx.ptote: Removable discontinuity (hole) - where the limit exists but the function is not defined. Jump discontinuity - where the limit doesn't exist because the left and right limits1are different. Wild oscillations discontinuity - where the function oscillates too much f(x) = sinlxmit: For removable discontinuities, we can remove the discontinuity by writing a related function filling in the undefined point: 1 f(x) = xsin x xsin 1 if x ≠ 0 becomes f(x) = { x 0 if x = 0 Iff(x) andg(x) are continuous at, thef(x)± g(x) ,f(x)g(x) , and(x),g(x) ≠ 0 are also continuousaa. g(x) 2 Determine wheref(x) = ln(x−1)+ 16−x is continuous: x −8 ln(x − 1) is continuous forx ∈ (1,∞) . √ 16 − x2 is continuous forx ∈ (−4,4) . 3 The denominator must not be 0,x − 8 ≠ 0 , ox ∈ (−∞,2)∪ (2,∞) . The function is continuous x ∈ (1,∞)∩ (−4,4)∩ ((−∞,2)∪ (2,∞)) , ox ∈ (1,2)∪ (2,4) . Continuity theorem Iff(x) is continuousx = b , anlimx→a g(x) = b , thelimx→a f(g(x)) = f(b). In other words, the limit can be moved within compositions. √−− −−−2 Evaluatelimx→0.5arcsin( 1 − x ) : −−−−−− −− −−−− lim arcsin( 1 − x ) = arcsin( lim √ 1 − x ) x→0.5 x→0.5 −−−−−−−−− 2 = arcsin(√ x→0.5 − x ) −−−−−−−−− 2 = arcsin(√ x→0.5 − x ) √ 3 = arcsin( ) 2 = π 3 Intermediate value theorem (IVT) Suppose f(x) is continuous ov[a,b]and f(a) < n < f(b). Then there exists a numc ∈ (a,b)such thaf(c) = n . In other words, for a function continuous on an interval, any value of the function in a range has a cox.esponding value of This is used most often to find the roots of complicated equatn = 0-.when This is an existance theorem - it tells us that a certain number with a certain property exists, but not its value. Prove thalnx = 3 − 2x has at least one real root. Constructf(x) = lnx − (3 − 2x) . We want to prove there exixsuch thatf(x) = 0. Clearly, this is continuoux > 0r. Constructa = 1,b = 2 (we could find these values using a graph). Clearlyf(x) is continuous over the interval. Clearlyf(1) = −1 andf(2) = ln2 − 1 . Clearlyf(1) < 0 andf(2) > 0 . So by the IVT, there must c ∈ [a,b]such thaf(c) = 0. So there must be a real root. This is a trancendental function and cannot be solved analytically. ;wip: prove this theorem Bisection method We can use the bisection to get a better approximation. Currently, we know th[1,2]. is in We can divide[a,b]into two equally sized intervals. Using IVT, we can determine that the root must be in one of the two intervals. So we have made the possible interval smaller, and therefore gotten a better estimate. This process can be repeated as many times as needed to get as good an estimate as needed. Limits at Infinity limx→∞ f(x) = L means ∀ϵ > 0,∃n > 0,x > n ▯ |f(x)− L < ϵ . So oncex exceeds some valuen,f(x) must stay withiϵ ofn. Heren has a role similarδ.o limx→−∞ f(x) = L means ∀ϵ > 0,∃n < 0,x < n ▯ f(x)− L < ϵ . Limit laws also applyx → ±∞ , including the squeeze theorem. 1 Prove limx→∞ k= 0,k > 0 : x Letϵ be an arbitrary positive real number. Constructn = k . √ϵ Assume x > n . Sox > 1. √ϵ So 1 < √ ϵand 1 − 0 < ϵ . x xk 1 Thereforelim x→∞ k= 0,k > 0 . x r 1 Infinite limit theoremr ∈ Q,r > 0 , whenx is definelimx→∞ xr = 0 . In other words, wreis rational and definedlim 1r= 0 . x→∞ x 2 Evaluatelimx→∞ x2+2 : 3x −4x 2 2 x +2 lim x + 2 = lim x2 x→∞ 3x − 4x x→∞ 3x −4x x2 x2 2 x2+ x2 = lim 2 x→∞ 3x2 − 42 x x 1 + 2 = lim x x→∞ 3 − 4 x 1 + 0 = 3 − 0 1 = 3 −−−2− −− Evaluatelimx→∞ ( 9x + x − 3x) : √ −−−−−− − = ( − 3x) + 3x −−−−−−− −−−−−− √ 9x + x + 3x lim( 9x + x − 3x) = lim( 9x + x − 3x) x→∞ x→∞ √ 9x + x + 3x 2 2 = lim 9x + x − 9x x→∞ √−9x + x + 3x = lim 1 x→∞ √9x +x x + 3 1 = lim −−−− x→∞ √ 9x +x+ 3 x2 1 = lim −−−−− x→∞ x (9x ) √ x2 + 3 1 = x→∞ √9 + 3 1 = 6 Evalualim −2 : x→−∞ x− x −2x Note that ∀x > 2,x > 2x lim −2 = −2 = −2 = 0 x→−∞ x −√ x − 2x −∞ − √ ∞ − ∞− −∞ Evalualimx→∞ e−xsinx: −1 ≤ sinx ≤ 1 1 e−x= ex lim ex= 0 x→∞ By the squeeze theorem: lim ⋅−1 ≤ lim e−xsinx ≤ lim ex⋅1 x→∞ x→∞ x→∞ 0 ≤ lim exsinx ≤ 0 x→∞ lim exsinx = 0 x→∞ 7/10/13 General techniquϵδfproofs: 1. Leϵbe an arbitrary positive real number. 2. Construct a parδas some functio. of 3. Show thδ > , so it's still in the domain. 4. Assum0 < x − a < δ. 5. Provf(x)− L < ϵ. 6. By the definition oflix→af(x) = L. A horizontal asymptote exy = Lif and onlimx→∞ f(x) = Lorlimx→−∞f(x) = L. 1 Considef(x) = e. Ax → 0 −,f(x) → 0. Butx → 0+ f(x) → ∞ . This is a one-sided asymptote - a vertical asymptote from the right. √x−x Evalualimx→1 : 1−√x 2 2 − − = ⋅ 2 2 lim √ x − x = lim √ x − x ⋅ 1 + √ x x→1 1 − √ x x→1 1 − √ x 1 + √ x 2 (√x − x )(1 + √ x) = lim x→1 1 − x √ x − x + x − x 2.5 = lim x→1 1 − x x(1 − x)+ √ x(1 − x ) = lim x→1 1 − x (1 − x)(x + √ x(1 + x)) = lim x→1 1 − x = lim(x + √ x(1 + x)) x→1 = 1 + √1(1 + 1)) = 3 Derivatives The derivative of a funcf(x) at the poiat, denotef(a): f(x)− f(a) f (a) = lim x→a x − a This can also be written as follows: f(x + h)− f(x) f (x) = lim h→0 h The first form is better for determining differentiability at a point, while the second is better for finding the derivative. If the limit exisa, the function is differentiaa.e at Two interpretations: f (x) is the slope of a tangef(x)fdrawn atx . ′ f (x) is the instantaneous rate of chanf(x). The following are equivalent notatf (x),y , dy, df, d f(x) . dx dx dx 1 xsin x ,x ≠ 0 Determine whetherf(x) = { 0,x = 0 : 1 ′ xsin x − 0 f (0) = x→0 x − 0 = limsin 1 x→0 x The limit does not exist 1 x sin x ,x ≠ 0 Determine whetherf(x) = { 0,x = 0 : 2 1 ′ x sin x − 0 f (0) = lim x→0 x − 0 1 = x→0xsin x = 0(squeeze theorem) Power law d dxx = nx n−1 . Proof for positive intn:er d n Consider dxx . Letn be a positive integer. (x+h − n By definition, x = lim h→0 ) x . dx h Using the binomial theorem: (x + h) − x = (x + nx n−1h + n(n−1)xn−2h + …+ nxh n−1 + h )− x n. (x+h) −x 2 n(n−1) Divide all termh:by = nx n−1+ xn−2h + …+ nxh n−2 + hn−1. hn(n−1) 2 n−2 n−1 Clearllimh→0(nx n−1+ 2 xn−2h + …+ nxh + h ) = nxx−1. d n n−1 So dxx = nx . Proof for all n(requires implicit differentiation, described later): n ln x nlnx Cledrln = ed nlnxe nlnx d So dxx = dxe = e ⋅dx (nlnx) . So d x = e nlnx⋅n⋅ 1= n xn= nx n−1. dx x x 9/10/13 −−−−− d √x + h − √ x √ x = lim dx h→0 −−−−− −−−−− √x + h − √ x √x + h + √ x = lim −−−−− h→0 h √x + h + √ x x + h − x = lim h→0 h(√x + h + √x) = lim 1 h→0√x + h + √ x 1 = 2 x √ Differentiability implies continuity Iff(x)is differentiabx = a , then it is also continuous at that point. Likewise, if a functiox = a,ot continuous at then it is also not differentiable - the contrapositive. Proof: f(x)is continuous x = aif and onlylimx→a f(x) = f(a), olimx→a(f(x)− f(a)) x−a = 0 . f(x)−f(a) f(x)−f(a) Clearly, this is equivlimx→ato x−a (x − a) = 0, olimx→a x−a limx→a(x − a) = 0. Clearly, this is equivf (a)⋅0 = 0 . ′ Clearly, this is true wf (a)eis defined. Sof(x) is continuousx = a only if (a)is defined - the function is diffex = aa.le at Some continuous functions are not differentiable. Considerf(x) = x . Clearf (0) = lim f(x)−f(0= lim ∣x∣= undefined . So the function is not differentiable x→0 x−0 x20 x atx = 0 due to a cusp. Others include piecewise funcxi.ns and ′ f(x)−f(0) 1 Considerf(x) = √ x. Clearf (0) = limx→0 x−0 = limx→0 2 = ∞ . So the function is not differentiable at x3 3 x = 0 due to a vertical tangent. Others arcsinx andx . Exponential functions x Considerf(x) = a . x+h x = − x+h x ′ a − a f (x) = lh→0 h x h a (a − 1) = lh→0 h x a − 1 = a lim h→0 h h So the derivative of any power function is just a power function multiplied by a constant of propolimoh→0ity−1 . h ′ a −1 Consider f (0) = lim h→0 h . Clearly, the constant of propertionality is the slope of thx = 0g.nt at e −1 We know that the slope of the tangentf(x) is 1 only whena = e. So we know thatlimh→0 h = 1 , and so d e = e x. dx Derivative Rules d Ifcis a constantdx c = 0. Sum/difference rule Iff(x) and g(x) are differentiablex,td (f(x)± g(x)) is defined xtand d (f(x)± g(x)) = d f(x)± d g(x) . dx dx dx dx Constant multiple rule Ifcis a constant anf(x) is differentiablex,td cf(x) is defined ax and d cf(x) = c d f(x) . dx dx dx Product rule d d ′ ′ Iff(x) and g(x) are differentiablex, thendx f(x)g(x) is defined xtand dx f(x)g(x) = f (x)g(x)+ f(x)g (x) . Proof: d f(x + h)g(x + h)− f(x)g(x) f(x)g(x) = lh→0 dx h f(x + h)g(x + h)− f(x)g(x)+ (f(x)g(x + h)− f(x)g(x + h)) = lim h→0 h f(x + h)− f(x) g(x + h)− g(x) = lim g(x + h) + f(x) ) h→0 h h f(x + h)− f(x) g(x + h)− g(x) = limg(x + h) + limf(x) h→0 h h→0 h = g(x)f (x)+ f(x)g (x)′ 11/10/13 2 x Differentiatx e : d 2 x x d 2 2 d x x 2 x (x e ) = e x + x e = 2xe + x e dx dx dx Reciprocal rule g (x) Ifg(x) is differentiablx , thend 1 is defined xtand d 1 = − . dx g(x) dx g(x) g(x) Proof: 1 1 − = 1 − 1 d 1 g(x+h) g(x) dx g(x) = h→0 h g(x)− g(x + h) = lim h→0 h(g(x + h)g(x)) g(x + h)− g(x) 1 = lim− ⋅ lim h→0 h h→0g(x + h)g(x) 1 = −g (x)⋅ 2 g(x) Quotient rule d f(x) d f(x) f (x)g(x)−f(x)g (x) If(x) andg(x) are differentiaxlandg(x) ≠ 0, thedx g(x)s definedxaand dxg(x)= g(x) Proof: d f(x) d −1 dx g(x) = dx (f(x)g(x) ) ′ −1 d −1 = f (x)g(x) + f(x) dx g(x) = f (x)g(x)−1+ f(x) d g(x)−1 dx −g (x) = f (x)g(x)−1+ f(x) g(x)2 ′ ′ f (x)g(x)− f(x)g (x) = 2 g(x) The functif(x) = x 2is known as a serpentine curve. Find the equations of the tangents where t:ey have a slope of 1+x 8 2 2 f (x) = 1 = 1 + x − 2x 8 (1 + x ) 1 1 − x2 = 2 8 (1 + x ) 22 2 (1 + x ) = 8 − 8x (1 + x ) − (8 − 8x ) = x + 10x − 7 = 0 −−−− −−−−−−− 2 −10 ± √100 + 4 ⋅1 ⋅7 x = −−−−−−−−−−− 128 x = ± √ −5 ± √ 4 −−−−−−− x = ± √ 4√2 − 5 −−−−−−− −−−−−−− 1 −−−−−−− t1(−√ 4√2 − 5) = f(−√ 4√2 − 5) = 8 √ 4√2 − 5 + b −−−−−−− −−−−−−− √ 4 2 − 5 1 −−−−−−− t1(−√ 4√2 − 5) = − = √ 4√2 − 5 + b −−−−−−− −−−−−−−−− 8 −−−−−−− −−−−−−− 2√ 4 2 − 5 √ ( 2 − 1)(4 2− 5) t1(− √ 4√2 − 5 ) = − − = b 8( 2 − 1) 8( 2 − 1) ;wip Higher order derivatives 2 The second order derivatf(x)is d2f(x) = d ( d f(x) = f (x) . dx dx dx It can be interpreted as the rate of change of the rate of change. In the same way, we can define arbitrarily high orders of derivatives, but this is rarely used. d d Consider dxas an operator. d2 x Find dx 1+x2: d 2 x d d x = dx 1 + x 2 dx dx 1 + x2 2 2 = d 1 + x − 2x dx (x + 1) 2 d 1 − x2 = 2 dx (x + 1) 2 −2x(x + 1) − (1 − x )(4x + 4x) = 4 (x + 1) 2 2 2 3 −2x(x + 1) − (1 − x )(4x + 4x) = 2 4 (x + 1) 2 2 2 2 = −2x(x + 1) − 4x(1 − x )(x + 1) 2 4 (x + 1) −2x(x + 1)− 4x(1 − x ) 2 = (x + 1) 3 16/10/13 Derivatives of Trigonometric and Hyperbolic Functions Considerlimx→0 sinx= 1 x ;wip: big ugly geometric proof, maybe find something better here: http://www.proofwiki.org/wiki/Limit_of_Sine_of_X_over_X sin(3x) Evaluatelimx→0 x : sin(3x) 3sin(3x) lim = lim x→0 x x→0 3x sin(3x) = 3 lim x→0 3x = 3 Sine rule d sin(x) = cos(x). dx Proof: sin(x + h)− sin(x) = d sin(x + h)− sin(x) sin(x) = lim dx h→0 h sin(x)cos(h)+ sin(h)cos(x)− sin(x) = lh→0 h = sin(x)lim cos(x)− 1 + cos(x)lim sin(h) h→0 h h→0 h cos(h)− 1 = sin(x)lim + cos(x)⋅1 h→0 h cos(h)− 1 cos(h)− 1 cos(h)+ 1 lim = lim ⋅ h→0 h h→0 h cos(h)+ 1 cos (h)− 1 = lim h→0h(cos(h)+ 1) 2 = lim 1 lim(1 − sin (h))− 1 h→0cos(h)+ 1 h→0 h 2 = 1 lim −sin (h) 2 h→0 h 1 sin(h) = ⋅− limsin(h)⋅ lim 2 h→0 h→0 h 1 = ⋅−0 ⋅1 = 0 2 d sin(x) = sin(x)⋅0 + cos(x)⋅1 dx = cos(x) Cosine rule d cos(x) = −sin(x). dx Proof: d cos(x + h)− cos(x) dx cos(x) =h→0m h cos(x)cos(h)− sin(h)sin(x)− cos(x) = h→0 h = cos(x)lim cos(h)− 1 − sin(x)lim sin(h) h→0 h h→0 h cos(h)− 1 = cos(x)lim − sin(x)⋅1 h→0 h cos(h)− 1 lim = 0, from the proof of the sine rule h→0 h d dx sin(x) = cos(x)⋅0 − sin(x)⋅1 = −sin(x) Tangent rule d 2 dx tan(x) = sec(x) Proof: d d sin(x) tan(x) = dx dx cos(x) cos(x) d sin(x)− sin(x)d cos(x) = dx dx cos(x)2 2 2 cos(x) + sin(x) = 2 cos(x) 1 = 2 cos(x) = sec(x)2 Hyperbolic sine rule d dx sinh(x) = cosh(x. Proof: d 1 d d sinh(x) = ( e − e−) dx 2 dx dx 1 d d = ( e − e−) 2 dx dx 1 −1 = ( e − x ) 2 e e + e−x = = cosh(x) 2 Hyperbolic cosine rule d cosh(x) = sinh(x. dx Proof: d 1 d x d −x dx cosh(x) = 2 ( dx e + dx e ) 1 d x d −x = 2 ( dx e + dx e ) = 1 (e + −1 ) 2 ex x −x = e − e = sinh(x) 2 Hyperbolic tangent rule d 1 dx tanh(x) = cosh(x)sech(x). Proof: d tanh(x) = d sinh(x) dx dx cosh(x) d d cosh(x)dx sinh(x)− sinh(x)dxcosh(x) = 2 cosh(x) cosh(x) − sinh(x)2 = cosh(x)2 = 1 = sech(x) cosh(x)2 Alternatively, we can prove it from the definition and derivative rules. 18/10/13 n n The triangle inequality statx + y ≤ x + y ,x ∈ R ,y ∈ R Chain Rule ′ ′ ′ d dg dh f (x) = g (h(x))h (x) =dxg ∘ h =dh dx. Proof: ;wip Simplify d cos(3x) : dx d d cos(3x) = −sin(3x) 3x = −3sin(3x) dx dx LetT(x) be the temperature at a height above the surface of the earth.h(t) be the height of a skydiver above the earth at timet. What does dT represent? dt Clearly,dT = dT dh . dT dt dh dt So dt is the rate of change in temperature with respect to height times rate of change in height with respect to time. So it is the rate of change in temperature with respect to height time velocity. Note that dT is independent of the skydiver - it is simply the change in temperature in the atmosphere. dh Likewise, dh is independent of the atmospheric temperature. dT dt So dt represents the rate of change in temperature with respect to time felt by the skydiver. Exponential Rule We can use the chain rule to find a x: dx x Clearly, d a = d eln(a )= d exlna. dx dx x ddx xlna d By the chain rule,dxa = dxlnae ⋅ dx xlna . d xlna d lna x x d x x Clearly,dxlna e ⋅ dxxlna = (e ) ⋅lna = a ⋅lna , sincedx e = e . So d a = a lnx dx Power Rule d n d n n−1 Ifg(x) is differentiablexa, thendx g(x) is defined xt and dx g(x) = n⋅g(x) ⋅g (x) . Proof: Let u = g(x),y = u n. Note that in this casu,is called an intermediate variable, since it is dependentxobut treated as an independent variable in dy. du Clearly, d g(x) = dy = dy du = nu n−1 u = ng(x) n−1 g (x) . dx dx du dx 21/10/13 Implicit Differentiation Useful when we do not have an explicit relation ly = f(x) . For example,x + y = 1 or y + 2x y + x = 1 4 . Useful for finding the derivatives of inverse functions. Contrasts with ordinary differentiation. This is differentiating for a general curve. dy Find dx for x + y = 1 : d d ( 2 + 2) = 1 d d (x + y ) = 1 dx dx d d d = x + y = 1 dx dx dx d 2 = 2x + y = 0 dx d 2 = 2x + y(x) dx d 2 d = 2x + y ⋅ y dy dx d = 2x + 2y y dx d 2x + 2y y = 0 dx d −2x −x dx y = 2y = y 3 3 Where does x + y = 3xy have a horizontal tangent? d 3 d 3 d x + y = 3xy dx dx dx = 3x + y 3y = 3y+ 3xy ′ 2 2 ′ 3x − 3y = (3x − 3y )y 2 2 ′ 3x − 3y = (3x − 3y )y ′ 3x − 3y x − y y = = = 0 3x − 3y 2 x − y 2 2 2 2 2 x − y = 0 ∧ x − y ≠ 0 ≡ y = x ∧ y ≠ √ x ≡ y = x ∧ x ≠ 0 ∧ y ≠ 0 3 2 3 2 3 6 3 3 3 x + (x ) = 3xx ≡ x + x = 3x ≡ x (x − 1) = 0 x = 0 (extraneous),x = 1 2 y = 1 = 1 There is a horizontal tangent at (1,1) We can use implicit differentiation to find the derivative of inverse functions. Considery = log a : y Clearlyx = a . So d x = d a = d a ⋅ dy. dx dx dy dx So 1 = a lna ⋅ dy. dy dx Clearly, = y1 . dx a lna ;wip: write this as a rule Derivative of Inverse Trignonometric Functions Arcsine rule d 1 dxarcsinx = √ 1−x2. Proof: Lety = arcsinx . Thenx = siny . dy Implicitly differentia1 = cosy dx . So dy = 1 . dx cosy We want to write this in termx.of −−−−−− −− Clearlycosy = ± √ 1 − sin y . π π We pick the positive answer sicosy ≥ 0 whenever − 2 ≤ y ≤ 2 , the rangearcsin . dy 1 So dx = 2 . √ 1−sin y So dy = 1 . dx √1−x2 Arccosine rule d arccosx = − 1 2 . dx √1−x Proof: Lety = arccosx . Thenx = cosy . Implicitly differentiatin1 = −siny dy . dx So dy = − 1 . dx siny We want to write this is terms ofx. −−− −−−−−2 Clearly,siny = ± √ 1 − cos y . We pick the positive answer since siny ≥ 0 whenever 0 ≤ y ≤ π , the range ofarccos . dy So = − 1 . dx √1−cos y dy 1 So dx = − √1−x 2. Arctangent rule d 1 dxarctanx = 2. 1+x Proof: Lety = arctanx . Then x = tany . dy Implicitly differentiatin1 = sec y . dy dx So = − 2 . dx sec y We want to write this in terms ofx . sin x cos x 1 Clearly,cos x + cos x = cos x. 2 2 Clearly,tan x + 1 = sec x . So dy = − 1 . dx tan x+1 So dy = − 1 . dx x +1 23/10/13 Logarithmic Differentiation Logarithmic differentiation is a technique useful for differentiating complicated products and quotients with powers, or h(x) functions of the form f(x) = g(x) . The general process is as fo
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