The word **Trigonometry **word derived from the Greek word

“ **Tri”** meaning “three “ ** “Gonia “** meaning an **“angle” **and **“Metron”** meaning **“measure” **thus

the word **“Trigonometry”** means **“three angle measure”** and

hence the literal meaning the measurement of a triangle.

v **T****rigonometry is a branch of mathematics which deals with relation between the sides and angles of a triangle with the help of a right angled triangle to determine un known heights and distances **

v **Trigonometry is very useful in navigation, engineering and astronomy.**

**Angle** is a measure of rotation of a given ray about its initial point.

The original ray is called the **initial side** and the final position of the ray

after rotation is called the **terminal side** of the angle.

The point of rotation is called the **vertex**.

If the direction of rotation is **anticlockwise**, the angle is said to be positive

and if the direction of rotation is **clockwise,** then the angle is negative

The measure of an angle is the amount of rotation performed to get the terminal side from the initial side.

There are several units for measuring angles.

**Angle :- **An angle is the union of two rays (sides) with common end point ”O”

The amount of rotation from the initial side to the terminal side is called the measure of the angle.

**Positive and negative Angles:-**

** **An angle formed by rotating ray is said to be positive or negative depending on whether it moves in an anti-clockwise or a clock wise direction, respectively together with the rotation in the plane necessary to bring one ray in the position of the ray.

v If the rotation is anti clockwise, the angle regards as **positive** and

v if clockwise the angle regarded as **negative**.

**An angle whose measure is more than 0 ^{0} but less than 90^{0} is called an Acute angle.**

**An angle whose measure is 90 ^{0} is called an Right Angle: **

**An right angle is exactly one quarter of a revolution.**

**An angle whose measure is more than 90 ^{0} but less than 180^{0} , is called an Obtuse Angle.**

**An angle whose measure is 180 ^{0}, is called an Straight angle.**

**A straight angle is exactly one half of a revolution.**

**An angle whose measure is more than 180 ^{0} but less than 360^{0} is called an Reflex Angle.**

**An angle whose measure is 360 ^{0}, is called an Complete Angle**

**If the terminal ray coincides with the initial ray after one complete rotation. Then the angle formed is called a complete angle.**

**If the terminal ray coincides with the initial ray, without any rotation. Then the angle formed is called ‘zero angle’.**

** **

**If the measure of the angle is 0 ^{0} it is called a zero angle.**