Find the equation of the tangent plane to the graph of f(x, y) = cos(2 x + y) at the point ( pi /2, pi /4, f( pi /2, pi /4)). Find the equation of the tangent plane at the point (2,1,1) to the graph of the function z = f(x, y) defined implicitly by xz + 2x2y + y2z3 = 11. Show that every tangent plane to the surface z2 = x2 + y2 passes through the origin.