一阶导数为Z'x=f'(x+y^2) -2x * f'(y-x^2)再继续对x 求偏导，得到x的二阶偏导数Z''xx= f''(x+y^2) -2f '(y-x^2) -2x *(-2x) *f''(y-x^2)=f''(x+y^2) -2f '(y-x^2) +4x^2 *f''(y-x^2)
