(a) If f(x, y) = ry, show that in the notation given, df = 2rydx + x dy. (b) Using the same notation and approach, write the differential of for the following functions: i. f(x, y) = ry cos(ry), ii. f(x, y, z) = x2yew/= iii. f(0, p) = sin?($) cos(@)