: 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