Angle is formed between two lines or rays and an angle is the measure of rotation between those lines. There are different types of angles in geometry. An angle which is less than 90° is known as an acute angle, an angle which is exactly equal to 90°is known as a right angle, and an angle greater than 90°is known as an obtuse angle. An angle that is exactly 180° is known as a straight angle and the angle which is greater than 180° is known as a reflex angle.
Example 1: Angles in a triangle are 45º, 45º and xº. What is the value of the third angle?
Sum of the angles in a triangle= 180º
This gives: 45º + 45º + xº = 180º
Now in order to solve for ‘x’, we can add the numbers first.
This implies: 90º + xº = 180º
Hence, xº = 180º - 90º = 90º
Therefore the measure of the third angle, x = 90º
Example 2: A line divides another straight line into two angles. If one part of the angle is 97°, then what is the other part of the angle on the straight line?
Given one part of the angle on the straight line = 97°
Let the other part of the angle on the straight line = x°
Total angle on a straight line = 180º
This implies: 97º + xº = 180º
Hence, xº = 180º - 97º = 83º
Therefore the measure of the other part of the angle, x = 83º