GOZUNTA
Try it now
All courses Math Foundations · MF.GM.4 51 of 60
Use angle facts and diagram structure to find unknown angle measures.

Use angle-pair totals

Problem
One angle in a supplementary pair measures 128°. What is the measure of the other angle?
Your answer
Show answer choicesAnswer choices Work through the mathematics first, then compare your reasoning.Select the answer that matches your work.
With a free account

Choose exactly what to practice.

Browse the learning objectives and included problem types, then build a session around the math you want to work on.

Create a free account
With paid access

Separate understanding from one lucky answer.

Multiple variants of a problem type let you apply the method again before deciding that the skill is secure.

Compare plans

Hint

Identify this as a supplementary pair, so use a total of 180°. Set up 180° − 128°.

Supplementary angles add to 180°, not 90°.

Solution walkthrough

01

Use the supplementary-angle fact

\[\text{supplementary}~\text{angles}~\text{sum}~\text{to}~180°\]

A supplementary pair forms a straight-angle total.

02

Write the equation

\[128°+x=180°\]

Combine the known angle and unknown angle into the supplementary total.

03

Solve

\[x=180°-128°=52°\]

Subtract the known measure from one hundred eighty degrees.

04

Check

\[128°+52°=180°\]

The two measures sum to a straight angle, confirming the solution.

05

Report the other angle

\[52°\]

The missing supplementary angle measures fifty-two degrees.

+

Another way

  1. Count from 128 to 180: add 2 to reach 130, then 50 more to reach 180, totaling 52.

!

Common mistake

Do not subtract from 360 degrees; supplementary angles sum to 180 degrees, not a full turn.