Supplementary Angles

Geometry is one of the important branches of mathematics that deals with the study of different shapes. It initiates the study of lines and angles. A straight line is a line without curves and it is defined as the shortest distance between two points. An angle is formed when the line segment meets at a point. There are different types of angles and the classification of pair of angles are given as

  • Complementary angles
  • Supplementary angles
  • Corresponding Angles
  • Linear pairs
  • Alternate angles
  • Vertically opposite angles

Here, let us discuss the pair of angle called supplementary angle in detail.

Supplementary Angles Definition

Two angles are said to be supplementary angles when their angles add up to 180 degrees. Two angles are supplementary, if

  • One of its angles is acute angle and another angle is an obtuse angle
  • Both of the angles are right angles.

This means that ∠A + ∠B = 180°.

Supplementary Angles

Difference between Complementary and Supplementary Angles

Complementary Angles Supplementary Angles
Two angles have a sum of 90° Two angles have a sum of 180°
∠A + ∠B = 90°. ∠A + ∠B = 180°.
Complementary angles form a right-angled triangle. Supplementary angles form a straight line.

Properties of Supplementary Angles

The important properties of supplementary angles are:

  • The two angles are said to be supplementary angles when they add up to 180°.
  • The two angles together make a straight line, but the angles don’t have to be together.
  • S” of supplementary angle stands for “Straight” line. This means that it forms 180°.

How to Find Supplementary Angles?

For example, if you had given that two angles form supplementary angles and you are provided with one angle and asked to find the other angle, you can easily find the other angle using the formula

∠A = 180° – ∠B (or)

∠B = 180° – ∠A

Adjacent Supplementary Angles

Adjacent Supplementary Angles

The supplementary angles may be classified as either adjacent or nonadjacent. The adjacent supplementary angle shares the line segment or arm with each other whereas the non-adjacent supplementary angles do not share the line segment or arm.

Supplementary Angles Theorem

The supplementary angle theorem states that if two angles are said to be supplementary to the same angle, then the two angles are said to be congruent.

Supplementary Angles Examples

Some of the examples of supplementary angles are

  • 120° + 60° = 180°
  • 90° + 90° = 180°
  • 140° + 40° =180°
  • 96° + 84° =180°

Solved Problem


Find the measure of unknown angle from the given figure.

Supplementary Angle Example


We know that the supplementary angle sum up to 180°

X + 55° + 40° = 180°

X + 95° = 180°

X = 180°- 95°

X = 85°

Therefore, the unknown angle, X = 85°

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