dz=fx(x,x^2)dx+fy(x,x^2)dy =﹙x^2-2x^3﹚dx+fy(x,x^2)dx²
dz=d﹙x³+1﹚=﹙x³+1﹚′dx=3x²dx
∴2xfy(x,x^2)=3x²-x²+2x³=2x³+2x²
fy(x,x^2)=x²+x
dz=fx(x,x^2)dx+fy(x,x^2)dy =﹙x^2-2x^3﹚dx+fy(x,x^2)dx²
dz=d﹙x³+1﹚=﹙x³+1﹚′dx=3x²dx
∴2xfy(x,x^2)=3x²-x²+2x³=2x³+2x²
fy(x,x^2)=x²+x