If (x ^ 2)/(a ^ 2 + u) + (y ^ 2)/(b ^ 2 + u) + (z ^ 2)/(c ^ 2 + u) = 1 then prove that ( partial u partial x )^ 2 +( partial u partial y )^ 2 +( partial u partial z )^ 2 =2(x partial u partial x +y partial u partial y +z partial u partial z )