See the point in the middle, at the origin, is a saddle point. If you look at the tangent plane to this graph, you will see that it is actually horizontal at the origin.
我们来看中间的那个点，在原点上，那就是一个鞍点，在这点做一个切平面,你可以看到这个切平面,是在原点上水平的。
Say you have a minimum, well, the tangent plane at this point, at the bottom of the graph is going to be horizontal.
如果你说这是个极小值，那么这点的切平面应该是水平的。
By horizontal tangent into the air filter layer, rotate and foreign get down to the oil, oil level of shock wave and flow surfaces by a foam, the air in the oil to get oil bath.
空气由水平方向切线进入过滤器的外来层，获得旋转并向下冲击油面，使油面产生波动并被气流掀起油沫，空气在此得到油浴。
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