## Definition of cosine

**:**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

**:**a trigonometric function cos θ that for all real numbers θ is given by the sum of a series of which the first term is 1 and succeeding terms alternate in sign with the

*k*th term consisting of a fraction whose numerator is θ raised to the exponent 2(

*k*- 1) and whose denominator is the factorial of 2(

*k*- 1) and that is exactly equal to the cosine of an angle of measure θ in radians

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