In trigonometry, either of two functions that complete each other, such as sine and cosine, which have a special complementary relationship.
Modern mathematical term: co- (together/complementary) + function (from Latin 'functio' meaning performance). Created in modern trigonometry to name the complementary relationships.
In trigonometry, sin and cos are cofunctions—sine of an angle equals cosine of its complement—and this complementary pairing extends to all the trig functions, creating a hidden harmony in mathematics that high school students usually miss.
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