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。

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The plane then made a promotional tour of 10 European and Middle Eastern capitals.

这架飞机当时在十个欧洲及中东国家首都做了推广之旅。

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The plane geometry is one of middle school mathematics basic contents.

平面几何是中学数学的基本内容之一。

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An Airbus spokesman told Forbes.com that although Middle Eastern airline Emirates had also shown interest in a stretched A380, the plane-maker was still "carefully observing" the market.

FORBES: Magazine Article