Trigonometry
is a branch of mathematics that means "measurement of, with and by means of triangles".

Astronomers as early as 150 BC developed the study of trigonometry. By about 1500 AD
the trig. ratios (sine, cosine and tangent) were established. Once again,
right-angled triangles, as in Pythagoras' Theorem, are important when making
calculations with the trig ratios.

Before we discuss the definition of
the trig ratios it is important to understand how angles are measured.

You are probably used to expressing angles in degrees,
but it is also possible to express angles in radians.

RADIANS A radian is the angle made when the radius of a circle
represents an arc on its perimeter.

OR

One radian is the measure of the angle subtended at
the centre of a circle by an arc equal in length to the radius of the circle.

Usually, 1 radian is written as 1^{c}.

1 radian = 57.30º
Radians are often used instead of degrees in advanced calculations.
Your calculator can work in radians by changing the mode of
operation from DEG to RAD.