The trigonometric functions are periodic wave functions that are used throughout math and physics. This page introduces and defines the functions using the geometry of the right triangle. See this page for the circle definitions of the functions.
The functions are defined below using the sides of the right triangle. Each function takes an angle of a right triangle as input and returns the ratio of two of its sides.
The functions sine, cosine and tangent are the most commonly used trigonometric functions. This page uses the degree angle system for measuring angles on the right triangle because degrees are often used to introduce the functions. However, in practice, the functions use the radian angle system as the default unit^{[1]}.
Given an angle of a right triangle the sine function returns the ratio of the opposite side over the hypotenuse of the triangle. For example, given an angle of the function returns the ratio . This is shown below.
Another example of a common trigonometric ratio is given by the 454590 right triangle shown below. To find the ratio, substitute the opposite and hypotenuse lengths into the definition.
Note, mathematicians don’t like having the square root operator in the denominator so they often rewrite the ratio by multiplying the expression by one in the form . This transformation is shown below.
Given an angle of a right triangle the cosine function returns the ratio of the adjacent side over the hypotenuse of the triangle. For example, given an angle of the function returns the ratio . This is shown below.
Another example of a common trigonometric ratio is given by the 454590 right triangle shown below. To find the cosine ratio, substitute the opposite and hypotenuse lengths into the definition.
Note, as discussed in the sine section mathematicians don’t like having the square root operator in the denominator so they often rewrite the ratio as shown below.
Given an angle of a right triangle, the tangent function returns the ratio of the opposite side over its adjacent side. For example, given the angle of the tangent function returns the ratio of .
As discussed in the sine section, mathematicians don’t like having the square root operator in the denominator so they often rewrite the ratio by multiplying by one as .

Radians Versus Degrees Concept