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这里。我们有一个三角形 ABC，看着像一个直角三角形。

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我们知道，它是一个直角三角形，

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因为 3的平方加上4 的平方等于 5 的平方。

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题目要求我们求出

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角ABC 的2倍的余弦是什么。

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这是角 ABC，

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我们不能马上给出它的值，

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但是我们知道角ABC 的余弦是什么，

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我们知道，角ABC 的余弦--

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00:00:27,810 --> 00:00:29,680
余弦就是邻边除以斜边。

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00:00:29,680 --> 00:00:32,159
它就等于 3/5。

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类似，我们知道角ABC的正弦是什么，

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它是对边除以斜边，

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它就是4/5。

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如果我们能把它分解为角 ABC 的余弦

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和正弦，那我们就能得到它的值。

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幸运的是，我们有一个三角恒等式

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正好能解决问题。

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我们知道一个角的2倍的余弦

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等于这个角的余弦的平方

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减去这个角的正弦的平方。

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