1. Acute angle:

An angle whose measure is from 0^{0} to 90^{0} is called an acute angle.

If x is an acute angle, then x < 90^{0}

2. Right angle:

An angle whose measure is 90^{0} is called a Right angle.

The square symbol denotes right angle, i.e. 90^{0}

If p is a right angle, then /p = 90^{0}

The two straight lines AB and AC are called perpendicular lines. It is denoted as below:

**AB ****│ AC**

Angle between two lines which are perpendicular to each other is 90^{0}

3. Obtuse angle

An angle whose measure is from 90^{0} to 180^{0} is called an obtuse angle.

If **p** is an obtuse angle, then **p** can be written as: 90^{0} < **p** < 180^{0}

4. Reflex angle

An angle whose measure is from 180^{0} to 360^{0} is called a reflex angle.

Let ‘a’ denote a reflex angle. Then, ‘a’ can be written as

**180 ^{0} < a < 360^{0}**

In the figure above for reflex angle, initial ray AB is rotated anticlockwise to reach the final ray position AC.

Reflex angle is formed when the initial ray AB is rotated beyond a straight line but within a full round. ** **

5. Straight angle

** **

An angle whose measure is 180^{0} is called a straight angle.

Rotate initial ray AB anti clockwise so that its position on AC, the final ray, points exactly in opposite direction of initial ray AB.

**6. Adjacent angles **

Two angles that have a common vertex and arm are called adjacent angles.

In the above figure, angles ** a** and

The two angles ** a** and

**7. Linear Pair of angles**

Two adjacent angles whose sum is 180^{0} are called linear pair of angles.

In the above figure, angles x and y are a pair of adjacent angles.

The sum of the measures of the two adjacent angles x and y is 180^{0}.

Therefore x and y form a linear pair of angles.

**Note: **

**All linear pair of angles are adjacent angles, but **

**All adjacent angles are not linear pair of angles. **

**8. Vertically opposite angles **

Two straight lines that intersect at a point form angles which are called vertically opposite angles.

Angles in the above figure are denoted by numerals 1, 2, 3 and 4.

The two straight lines AD and BC intersect each other at point P.

P is called the point of intersection of the two straight lines AD and BC.

In the above figure, there are two pairs of vertically opposite angles. One pair is angles 1 and 4. The other pair is angles 2 and 3.

Now,

and also

**Also note that: **

and

and

**9. Complementary angles **

Two angles whose sum is 90^{0} are called complementary angles.

In the figure above

**Note: **

**If p is one angle, then its complement is 90 ^{0} – p **

**10. Supplementary angles **

Two angles whose sum is 180^{0} are called supplementary angles.

In the above figure, angles 1 and 2 are supplementary angles.

Also, they form a linear pair of angles.

**Note: **

If ** x** is one angle, then its supplement is

**11. What is the sum of all of the angles at a point?**

In the above figure, the three straight lines AC, AB and AD intersect each other at point A.

Around vertex A, three angles are formed. They are named **X, Y and Z.**

Now,