[m]\frac{ ∂^2z }{ ∂x ∂y}=(\frac{ ∂z }{ ∂y })`_{x}=(-x^2\cdot cos(x-y))`_{x}=-2x\cdot cos(x-y)-x^2]\cdot (-sin(x-y))\cdot (x-y)`_{x}=-2x\cdot cos(x-y)+x^2\cdot sin(x-y)[/m]
Подставляем в уравнение:
[m]-2x\cdot cos(x-y)+x^2\cdot sin(x-y)-\frac{2}{x}\cdot (-x^2\cdot cos(x-y))-x^2\cdot sin(x-y)=0[/m] - верно
[m]-2x\cdot cos(x-y)+x^2\cdot sin(x-y)+2x\cdot cos(x-y)-x^2\cdot sin(x-y)=0[/m] - верно
0=0