(adsbygoogle = window.adsbygoogle || []).push({}); An angle -- why an angle is the ratio of two sides of a triangle?

Perhaps I should have already learned this, but I just can't seem to grasp why an angle is the ratio of two sides of a triangle, or the length of a circular arc by its radius.Why is this so?

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# An angle - why an angle is the ratio of two sides of a triangle?

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