Question

高数u=xy^2+z^3 求div(grad(u))=?

Answer 1

grad(u)

=(∂u/∂x,∂u/∂y,∂u/∂z)

=(y^2,2xy,3z^2),

所以

div(grad(u))

=div(y^2,2xy,3z^2)

=∂(y^2)/∂x+∂(2xy)/∂y+∂(3z^2)/∂z

=0+2x+6z

=2x+6z