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Supplementary Angles

Two angles that add to 180 degrees.

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Supplementary Angles

What if you were given two angles of unknown size and were told they are supplementary? How would you determine their angle measures? After completing this Concept, you'll be able to use the definition of supplementary angles to solve problems like this one.

Watch This

CK-12 Basic Geometric Definitions

Watch this video beginning at around the 3:20 mark.

James Sousa: Angle Basics--Supplementary Angles

Then watch the second part of this video.

James Sousa: Supplementary Angles


Two angles are supplementary if they add up to 180. Supplementary angles do not have to be congruent or adjacent.

Example A

The two angles below are supplementary. If mMNO=78 what is mPQR?

Set up an equation. However, instead of equaling 90, now the sum is 180.


Example B

What are the measures of two congruent, supplementary angles?

Supplementary angles add up to 180. Congruent angles have the same measure. So, 180÷2=90, which means two congruent, supplementary angles are right angles, or 90.

Example C

Find the measure of an angle that is a supplementary to MRS if mMRS is 70.

Because supplementary angles have to add up to 180, the other angle must be 18070=110.


Guided Practice

Find the measure of an angle that is supplementary to ABC if mABC is:

  1. 45
  2. 118
  3. 32
  4. 2


  1. 18045=135.
  2. 180118=62.
  3. 18032=148.
  4. 1802=178.

Explore More

Find the measure of an angle that is supplementary to ABC if mABC is:

  1. 114
  2. 11
  3. 91
  4. 84
  5. 57
  6. x
  7. (x+y)

Use the diagram below for exercises 8-9. Note that NK¯¯¯¯¯¯IL.

  1. Name two supplementary angles.
  1. If mINJ=63, find mJNL.

For exercise 10, determine if the statement is true or false.

  1. Supplementary angles add up to 180.

For 11-12, find the value of x.

Answers for Explore More Problems

To view the Explore More answers, open this PDF file and look for section 1.8. 


Supplementary angles

Supplementary angles

Supplementary angles are two angles whose sum is 180 degrees.

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