Find ∂z∂u and ∂z∂v if z=f(x,y)=x2+xy−y2 and x=e2u+v and y=lnvu.
Solution The function z=f(x,y) is a composite function of two independent variables u and v. So, we use Chain Rule II. ∂z∂u=∂z∂x∂x∂u+∂z∂y∂y∂u
Since ∂z∂x=2x+y∂z∂y=x−2y∂x∂u=2e2u+v∂y∂u=∂∂u(lnv−lnu)=−1u
we have ∂z∂u=(2x+y)⏟∂z∂x(2e2u+v)⏟∂x∂u+(x−2y)⏟∂z∂y(−1u)⏟∂y∂u=(2e2u+v+lnvu)(2e2u+v)−(e2u+v−2lnvu)(1u)x=e2u+v;y=lnvu
Similarly, since ∂x∂v=e2u+v and ∂y∂v=1v, we have ∂z∂v=(2x+y)⏟∂z∂x(e2u+v)⏟∂x∂v+(x−2y)⏟∂z∂y(1v)⏟∂y∂v=(2e2u+v+lnvu)(e2u+v)+(e2u+v−2 lnvu)(1v)x=e2u+v;y=lnvu