題目:Ａ unit normal vector for the surface z=sin(xy) at the point (1,0,0)is?
解答是z=sin(xy) 令F(x,y,z)=z-sin(xy)
則梯度＝(-ycos(xy),-xcos(xy),1)
再帶入點(1,0,0) 答案為(0.-1,1)
單位向量（0,-1/根號2,1/根號2）
我想問的是為什麼不能用F(x,y,z)=sin(xy)-z 呢？
這樣算出來的單位向量是(0,1/根號2,-1/根號2)