Zài zhíjiǎo sānjiǎoxíng lǐ, yí ge ruìjiǎo de yúxián děngyú tā de línbiān bǐ xiébiān.
In a right triangle, the cosine of an acute angle equals the adjacent side over the hypotenuse.
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余 means "what remains": the cosine of an angle is the sine of what remains of a right angle, just as English co- stands for "complement". The set: 正弦 sine, 余弦 cosine, 正切 tangent, 余切 cotangent. The law of cosines is the 余弦定理.
