Consider the following. f(x, y) = 4x - 3y (a) Find (4, 6) and f(4.1. 6.05) and calculate Az. f(4, 6) = (4.1, 6.05) = Az = (b) Use the total differential dz to approximate Az. dz= Consider the following. (Round your answers to four decimal places.) f(x, y) = yex (a) Find f(2, 1) and f(2.8, 1.85) and calculate Az. f(2, 1) = f(2.8, 1.85) = Az = (b) Use the total differential dz to approximate Az. dz=