The cosine function is one of the three basic functions used in trigonometry. The cosine of a right triangle is found by taking the ratio of the length of the triangle's adjacent side over the length of the hypotenuse. In other words, divide them.
The cosine is often used in conjunction with the other basic trigonometry functions, which are the sine and the tangent. A common mnemonic makes it easier to remember the definitions and formulas of the cosine, the sine and the tangent. The mnemonic is SOHCAHTOA, which means Sine equals Opposite over Hypotenuse, Cosine equals Adjacent over Hypotenuse, Tangent equals Opposite over Adjacent.