如果你的脚和你伸直的腿成直角，且没有晃荡，那么说明你有一个较低的踝关节，你穿平底鞋会比穿高跟鞋更舒适。
If your foot is at a right Angle to your outstretched leg and does not dangle, you have a low ankle-joint axis and will be more comfortable in flat shoes than in high heels.
我们在直角坐标系中处理，在这里dV有可能变成，或者它们的其他排列。
Well, we can do that in rectangular coordinates dz dx dy where dV becomes something like, maybe, dz dx dy, or any permutation of these.
两个有桅的船成直角的垂在前桅并且船头和船尾在主桅上。
Two-masted vessel square-rigged on the foremast and fore - and-aft rigged on the mainmast.
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