u=x^(y/z)∂u/∂x = (y/z)x^(y/z-1),∂u/∂y = x^(y/z)lnx*(1/z) = (1/z)x^(y/z)lnx,∂u/∂z = x^(y/z)lnx*(-y/z^2) = (-y/z^2)x^(y/z)lnx.
