Given the right triangle $$ABC$$, we define the sine, the cosine and the tangent of an angle $$x$$ as:

$$\sin(x)$$: the sine of the angle $$x$$ is the ratio between the opposite cathetus and the hypotenuse: $$$ \sin(x)=\dfrac{a}{c} $$$

$$\cos(x)$$: the cosine is the ratio between the adjoining cathetus and the hypotenuse: $$$ \cos(x)=\dfrac{b}{c} $$$
 $$\tan(x)$$: the tangent is defined as the ratio between the opposite cathetus and the adjoining one: $$$ \tan(x)=\dfrac{a}{b} $$$