If u=(xz)2+(yz)2+(xy), then Σx∂u∂x=
∂u∂x=2xz2+1y∂u∂y=2yz2−xy2∂u∂z=−2x2z3−2y2Z3∑x∂u∂x=x∂u∂x+y∂u∂y+z∂u∂z =0
If u=log(x3+y3+z3−3xyz) and (∂∂x+∂∂y+∂∂z)2u=−k(x+y+z)2