12. Find the derivative of f at Po and in the direction of Ã: (a) f(x,y) = 2xy-3y², Po(5,5), A=4i+3j (b) f(x, y) = tan¯¹(y/x) + √√3 sin¹ (2724), Po(1,1), A = 3i – 2j (c) f(x, y, z) = x²+2y2-322, Po(1, 1, 1), A=i+j+k