How to Calculate the Missing Angle in a Right Triangle

Though the angles of a triangle are located inside of it, they do not have to remain a mystery. A triangle, also called a trigon, is a three-sided polygon. One feature that every triangle shares is that the sum of its three interior angles equals 180 degrees. A right triangle is a one in which one of its interior angles is a right angle, which means it measures 90 degrees. If you know one of the other angles of the triangle, finding the third angle is easy.

Things You'll Need

• Calculator

Instructions

1. Find the two known angles. In this example, one angle is 90 degrees since the triangle is a right triangle. For purposes of this example, the other angle is 55 degrees.

2. Sum the two known angles. In this example, 90 added to 55 equals 145.

3. Subtract the sum of angles from 180. Concluding this example, 155 subtracted from 180 results in 35. Therefore, the third, previously unknown angle is 35 degrees.

References

• Wolfram MathWorld: Triangle
• Math Open Reference: Right Triangle
• Interactive Maths Series Software: Angle Sum of a Triangle

Resources

• 1728 Software Systems: Ultimate Triangle Calculator