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Cosine

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

$\begin{array}{cccc}\text{cosine}=\frac{\text{length of the leg adjacent to the angle}}{\text{length of hypotenuse}}& & & \text{abbreviated "cos"}\end{array}$

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,

$\text{secant}=\frac{\text{length of the hypotenuse}}{\text{length of the leg adjacent to the angle}}$ .

In the above example, $\sec \left(A\right)=\frac{5}{4}$ and $\sec \left(C\right)=\frac{5}{3}$ .

See also: sine and tangent .