If R rightarrow = A rightarrow - Brightarrow, show that R2 = A2 + B2 - 2 A B Cos theta, where theta is the angle between Arightarrow and Brightarrow. Determine the value of n, so that the vectors Arightarrow = i + 5j + nk and Brightarrow = 2i - j + k are perpendicular.