Suppose f(x, y, z) and g(x, y, z) are continuously differentiable functions with f(1, 2, 3) = 5 and g(1, 2, 3) = 7. assume that the surface f(x, y, z) = 5 and the surface g(x, y, z) = 7 have the same tangent plane at (1, 2, 3). evaluate ∇f(1, 2, 3)×∇g(1, 2, 3) (remember to give reasons).