That means, if you are going in a plane and you throw something up in the air, it seems to be going up and down to you.

也就是说,如果你在飞机中,你向空中扔出一物体,它也许会相对于你上下运动

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And similarly, actually, if we're looking at our polar coordinates here, what we see is it's any place where theta is equal to is what's going to put up on the x-y plane.

类似的，如果我们,看这里的极坐标系，我们能看到只要在theta等于,多少的地方就是ｘｙ平面。

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