Cosine means "sine of the complement."  Explain why this is a logical name for cosine.

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beckden | High School Teacher | (Level 1) Educator

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The compliment of an angle is `90-theta` (or `pi/2-theta` ), because if `alpha+beta=90` then `alpha` and `beta` are complimentary.

`sin(90-theta )=cos(theta)`

So the sine of the compliment is the cosine.

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