HESI Math Practice Exam 2026 - Free Math Practice Questions and Study Guide

In a triangle, the sum of all interior angles is always 180 degrees. Given that two of the angles in this triangle measure 90 degrees and 45 degrees, you can find the third angle by following these steps:

First, add the measures of the known angles together:

90° + 45° = 135°

Next, subtract this sum from the total degrees in a triangle:

180° - 135° = 45°

This result shows that the measure of the third angle must be 45 degrees. Therefore, the correct answer is indeed 45°. It aligns with the fundamental rule of triangles regarding the sum of angles, illustrating how to apply this principle practically.