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EXAMPLE 4Differentiating a Function of Two Variables Where Each Variable Is a Function of Two Variables

Find zu and zv if z=f(x,y)=x2+xyy2 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. zu=zxxu+zyyu

Since zx=2x+yzy=x2yxu=2e2u+vyu=u(lnvlnu)=1u

we have zu=(2x+y)zx(2e2u+v)xu+(x2y)zy(1u)yu=(2e2u+v+lnvu)(2e2u+v)(e2u+v2lnvu)(1u)x=e2u+v;y=lnvu

Similarly, since xv=e2u+v and yv=1v, we have zv=(2x+y)zx(e2u+v)xv+(x2y)zy(1v)yv=(2e2u+v+lnvu)(e2u+v)+(e2u+v2 lnvu)(1v)x=e2u+v;y=lnvu