Definition of cosine
1
: a trigonometric function that for an acute angle is the ratio between the leg adjacent to the angle when it is considered part of a right triangle and the hypotenuse
2
: a trigonometric function cos θ that for all real numbers θ is given by the sum of the alternating series {latex}1  \frac{x^2}{2!} + \frac{x^4}{4!}  \frac{x^6}{6!} + \frac{x^8}{8!}  \dots{/latex} and that is exactly equal to the cosine of an angle of measure θ in radians
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Time Traveler for cosine
The first known use of cosine was in 1635
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