Verify the first special case of the chain rule for the composition f∘c in each of the cases:
(a) f(x, y)=xy, c(t)=(et, cost)
(b) f(x, y)=exy, c(t)=(3t2,t3)
(c) f(x, y)=(x2+y2)log√x2+y2,c(t)=(et, e−t)
(d) f(x, y)=xexp(x2+y2),c(t)=(t,−t)