并且这个平面包含了原点和动点，以及速度矢量。
And, that's the plane that contains the origin, the point, and the velocity vector.
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。
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.
因为所有的平面都过原点。
Well, that's because all of your planes have to pass through the origin.
因此，在笛卡尔平面使用x轴时，考虑一个特定的细胞为原点。
So, as you do with the x-axis in the Cartesian plane, you consider a particular cell to be the origin.
极坐标是关于r，θ的,用r，θ代替平面上一点的x，y坐标，是从原点到那一点的距离，，which，is，the，distance，from，the，origin，to，a，point，θ是由x轴逆时针旋转,而得到的夹角。
Remember that polar coordinates are about replacing x and y as coordinates for a point on a plane by instead r, r and theta,which is the angle measured counterclockwise from the positive x-axis.
极坐标是关于r，θ的,用r，θ代替平面上一点的x，y坐标，是从原点到那一点的距离，，which，is，the，distance，from，the，origin，to，a，point，θ是由x轴逆时针旋转,而得到的夹角。
Remember that polar coordinates are about replacing x and y as coordinates for a point on a plane by instead r, r and theta,which is the angle measured counterclockwise from the positive x-axis.
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