There are three basic ways to define the trigonometric functions.
Consider a point
(x, y)
on the terminal side of an angle
θ
in
standard position. It lies a distance
d
away from the origin.
trigfunctions
sine(θ) = sin(θ) = .


cosine(θ) = cos(θ) = .


tangent(θ) = tan(θ) = .


cosecant(θ) = csc(θ) = .


secant(θ) = sec(θ) = .


cotangent(θ) = cot(θ) = .


The unit circle is the circle with equation
x
^{2} + y
^{2} = 1
. Let
s
be the
length of the arc with one endpoint at
(1, 0)
extending around the circle
counterclockwise with its other endpoint at
(x, y)
. Note that
s
is both the
length of an arc as well as the measure in radians of the central angle that
intercepts that arc.
trigfunctions2
tangent(s) = tan(s) = ;;x≠0.


cosecant(s) = csc(s) = ;;y≠0.


secant(s) = sec(s) = ;;x≠0.


cotangent(s) = cot(s) = ;;y≠0.


Consider a right triangle with one acute angle
θ
in standard position.
Let the side opposite that angle be called the opposite side. Let the other leg
be called the adjacent side.
trigfunctions3
sine(θ) = sin(θ) =


cosine(θ) = cos(θ) =


tangent(θ) = tan(θ) =


cosecant(θ) = csc(θ) =


secant(θ) = sec(θ) =


cotangent(θ) = cot(θ) =


The trigonometric functions have different signs according to the quadrant
in which the angle's terminal side lies. Here is a chart to show these signs.
Figure %: The signs of the trigonometric functions in the quadrants
The quadrantal angles have the values as shown in the chart below.