Example

Finding the Unknown Angle of a Right Triangle

Apply the geometry problem-solving strategy to find the unknown acute angle in a right triangle when one acute angle is given.

Problem: One angle of a right triangle measures 28°28°. What is the measure of the third angle?

  1. Read and draw: Sketch a right triangle ABC\triangle ABC and label the right angle as 90°90°, the known acute angle as 28°28°, and the unknown angle as 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:

mA+mB+mC=180m\angle A + m\angle B + m\angle C = 180

Substitute the known values:

x+90+28=180x + 90 + 28 = 180

  1. Solve: Combine the constants: x+118=180x + 118 = 180. Subtract 118118 from both sides: x=62x = 62.
  2. Check: 90+28+62=?18090 + 28 + 62 \stackrel{?}{=} 180180=180180 = 180 \checkmark
  3. Answer: The measure of the third angle is 62°62°.

Because a right triangle always has a 90°90° angle, only one additional angle measurement is needed to determine the third. The angle sum equation reduces to x+90+(known angle)=180x + 90 + (\text{known angle}) = 180, making the setup particularly straightforward.

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Updated 2026-04-21

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