Example

Finding the Third Angle of a Triangle Given Two Angles

Apply the geometry problem-solving strategy to find an unknown angle when two angles of a triangle are given.

Problem: The measures of two angles of a triangle are 55°55° and 82°82°. Find the measure of the third angle.

  1. Read and draw: Sketch a triangle and label the known angles 55°55° and 82°82°; label the unknown angle xx.
  2. Identify: The measure of the third angle.
  3. Label: Let xx = the measure of the unknown angle.
  4. Translate: Apply the angle sum property:

55+82+x=18055 + 82 + x = 180

  1. Solve: Combine the known values: 137+x=180137 + x = 180. Subtract 137137 from both sides: x=43x = 43.
  2. Check: 55+82+43=?18055 + 82 + 43 \stackrel{?}{=} 180180=180180 = 180 \checkmark
  3. Answer: The measure of the third angle is 43°43°.

This example illustrates how the triangle angle sum property converts a geometry question into a simple one-step linear equation: add the two known angles, then subtract from 180180.

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Updated 2026-04-21

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