Ответ:          d²u/dxdy = cos(xy) - xy sin(xy) .

Объяснение:

u(x,y)=sin(x*y) ;

du/dx = cos(x*y) *( xy )' = y cos(x*y);

d²u/dxdy = (  y cos(x*y) )' = 1*cos(xy) - y *sin(xy) *x = cos(xy) - xy sin(xy) .