Understanding how to find a missing angle in a triangle is a key geometry skill for upper‑elementary and middle‑school students. This set includes four free printable worksheets that give students practice with right triangles, isosceles triangles, equilateral triangles, and scalene triangles.
Answer keys are included for all worksheets.
How to Find a Missing Angle in a Triangle
All triangles have interior angles that add up to 180°. To find a missing angle:
- Add the two known angles.
- Subtract that total from 180°.
- The result is the missing angle.
This rule works for every type of triangle:
- Right triangles — one angle is always 90°.
- Isosceles triangles — two angles are equal.
- Equilateral triangles — all angles are 60°.
- Scalene triangles — all angles are different.
Example Problem
A triangle has angles of 40° and 65°. What is the missing angle?
40° + 65° = 105°
180° − 105° = 75°
The missing angle is 75°.
Free Printable Worksheets
Each worksheet includes a variety of triangle types and increasing difficulty levels.
Worksheet 1 Worksheet 2 Worksheet 3 Worksheet 4These worksheets are ideal for 4th‑grade geometry, 5th‑grade math, and middle‑school review. They work well for homeschool lessons, classroom warm‑ups, independent practice, and general test preparation. Each worksheet is designed to be simple, clear, and easy to print.
Why This Skill Matters
Finding missing angles helps students build:
- Understanding of geometric rules
- Problem‑solving skills
- Algebra readiness
- Confidence with multi‑step reasoning
It’s a foundational concept that appears in many later math topics.
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