Math Homework. Do It Faster, Learn It Better.


The cosine of an angle is the trigonometric ratio of the adjacent side to the hypotenuse of a right triangle containing that angle.

cosine = length of the leg adjacent to the angle length of hypotenuse abbreviated "cos"


In the triangle shown, cos ( A ) = 4 5 and cos ( C ) = 3 5

The cosine ratio is the same regardless of the size of the right triangle. So, it is often easiest to consider a right triangle with a hypotenuse of length 1 .

The cosine ratio can also be thought of as a function, which takes different values depending on the measure of the angle. You can measure an angle in degrees or radians .

The reciprocal of the cosine ratio is known as secant ratio of an angle, abbreviated as "sec".

That is,

secant = length of the hypotenuse length of the leg adjacent to the angle .

In the above example, sec ( A ) = 5 4 and sec ( C ) = 5 3 .

See also: sine and tangent .