阿摩線上測驗
題組內容
4. The function f(x,y, z) has continuous partial derivatives. Assume that point (1,2,-1) lies on the level surtace S : f(x,y, z) = 5 and the tangent plane of S at (1,2, -1) is -x+y+3z = -2. Assume that at (1,2,-1) along the vector 
the directional derivative 