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Supplementary Angles

Two angles that add to 180 degrees.

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Supplementary Angles

Supplementary Angles

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

What if you were given two angles of unknown size and were told they are supplementary? How would you determine their angle measures?




Example 1

Find the measure of an angle that is supplementary to ABC if mABC is 118.


Example 2

Find the measure of an angle that is supplementary to ABC if mABC is 32.


Example 3

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 4

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 5

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.


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.

Review (Answers)

To see the Review answers, open this PDF file and look for section 1.8. 



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Supplementary angles

Supplementary angles are two angles whose sum is 180 degrees.

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