我的意思是，上一次我们讨论过了，这个梯度是垂直于等高线，并且是指向方程增长的方向的。
I mean we'd seen in last time, but the gradient is perpendicular to the level and points towards higher values of a function.
梯度向量的垂直方向，为什么是这样而不是另外一个方向？
Does the gradient vector, why is the gradient vector perpendicular in one direction rather than the other?
垂直于梯度的方向上,方向导数为零。
The directional derivative in a direction that's perpendicular to the gradient is basically zero.
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