直接把y看作是常数,
对x求导
z(x,y)=[cos(xy)]^3+[sin(xy)]^3
Zx(x,y)=3[cos(xy)]^2*[cos(xy)]'(xy)'+3[sin(xy)]^2*[sin(xy)]'(xy)'
=3y[(sin(xy)^2(cos(xy))+cos(xy)^2(-sin(xy))]