所以记住，在上次课还是第一节的时候我们知道，要把一个向量旋转90度，我们要把变成。

So, remember: what we saw last time or in the first lecture is that, to rotate a vector by 90 degrees, we should change to.

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弗兰在第二节开始时命中一个旋转的任意球，追平比分，乌拉圭最终也以罚点球取胜。

Forlan tied it with a swirling free kick early in the second half, then penalty kicks won it for Uruguay.

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四元数就不会遭受万向节锁的痛苦，因为它不用三个分离的轴表示旋转。

Quaternions do not suffer from gimbal lock, because they do not express rotations relative to three separate axes.

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