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 5555^{\circ} and 8282^{\circ}. Find the measure of the third angle.

  1. Read and draw: Sketch a triangle and label the known angles 5555^{\circ} and 8282^{\circ}; 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 triangle angle sum property: 55+82+x=18055 + 82 + x = 180.
  5. Solve: Combine the known values: 137+x=180137 + x = 180. Subtract 137 from both sides: x=43x = 43.
  6. Check: Verify the sum: 55+82+43=18055 + 82 + 43 = 180, which is true.
  7. Answer: The measure of the third angle is 4343^{\circ}.

This example illustrates how the triangle angle sum property converts a geometry question into a simple one-step linear equation.

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Updated 2026-06-25

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