How to Find the Missing Measure of a Triangle


Finding the missing internal angle in a triangle is a relatively simple math task that just about anyone can perform. There is a simple equation that you can use to determine the angle accurately every time without fail. Once you learn the equation, you can properly apply it toward solving for the unknown. You can further apply the knowledge you utilize in solving the math problem to real-world applications.

  • Determine the appropriate equation. Given two out of three internal measurements of a triangle, you can solve for the missing side with the following equation: A+B+C=180. A, B and C stand for the three angles within the triangle.

  • Insert the known values into the equation. Assign a letter to the three angles in the triangle and insert them into the equation. For example, if the two measurements you know are 90 degrees and 35 degrees, you could assign the letter A to 90 and B to 35. Inserting them into the equation results in the following: 90+30+C=180. C stands for the missing, or unknown internal angle.

  • Add together the known values. In our example, 90 plus 30 is 120 so 120+C=180.

  • Solve for C. To solve for the missing angle, subtract the total of the known angles from 180. In our example, 180 minus 120 is 60, so C=60. The missing angle in the triangle is 60 degrees.


  • Photo Credit triangle image by Zbigniew Nowak from
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