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Triangle Angle Sum Theorem Calculator

Online triangle angle-sum calculator. Enter two interior angles to find the third (the angles sum to 180°), with the theorem—great for elementary geometry.

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Interior angles of a triangle sum to 180°
CAB
Both angles must be >0
Their sum must be <180

📖 Tutorial | Triangle Angle Sum

1. Definition
The three interior angles sum to 180°. Given two, the third = 180° − their sum.
2. Formula & Notation
A, BThe two known angles
CThird angle, 180−A−B
3. Properties & Laws
  • Interior sum is always 180°;
  • Right triangle acute angles sum to 90°;
  • Equilateral angles are 60°;
  • Exterior angles sum to 360°.
4. Steps
  1. Enter two angles A, B (>0, sum<180);
  2. Click "Find Third";
  3. Or click "Load Example".
5. Worked Examples
Example: angles 60° and 70°.
Solution: C = 180−60−70 = 50°.
6. Common Pitfalls
The two angles must sum to less than 180°;
Angles must be positive;
Not the exterior sum 360°.

❓ FAQ | Triangle Angle Sum

What do interior angles sum to?
Always 180°.
How to find the third angle?
Subtract the two angles from 180.
Right triangle?
Acute angles sum to 90°.
For 60°,70°, third?
50°.
Equilateral angles?
Each is 60°.
Exterior angles?
They sum to 360°.