The cosine of zero is one. The cosine of an angle is calculated by dividing the length of the side of a right triangle that is adjacent to the acute angle by the length of the hypotenuse.
For a zero-degree angle, the hypotenuse and the adjacent side of the right triangle are the same line, and when a number is divided by itself, the resulting quotient is one. The lengths of the sides of the right triangle used in the function cosine are calculated by using the Pythagorean theorem. The sine of a zero-degree angle has a zero in the numerator and is infinite.