题目: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)
则梯度＝(-ycos(xy),-xcos(xy),1)
再带入点(1,0,0) 答案为(0.-1,1)
单位向量（0,-1/根号2,1/根号2）
我想问的是为什么不能用F(x,y,z)=sin(xy)-z 呢？
这样算出来的单位向量是(0,1/根号2,-1/根号2)