Every triangle's interior angles sum to 180°, and every quadrilateral's to 360°!
No matter the shape or size, the interior angles of ANY triangle always sum to exactly 180°. For quadrilaterals (4-sided polygons), they always sum to 360°. These facts let you find any missing angle if you know the others.
Triangle angles: sum = 180°. Quadrilateral angles: sum = 360°.
A triangle has angles of 50° and 70°. Third angle = 180° − 50° − 70° = 60°. Check: 50° + 70° + 60° = 180° ✓
A quadrilateral has angles of 90°, 85°, and 110°. Fourth angle = 360° − 90° − 85° − 110° = 75°. Check: 90 + 85 + 110 + 75 = 360° ✓
Types of triangles by angle:
Regardless of type, all three interior angles always total 180°!
Finding a missing interior angle:
A triangle has two angles measuring 45° and 80°. What is the third angle?
Remember This!
A triangle can never have two right angles — that would already equal 180° with no room for the third! And a triangle with an obtuse angle must have two acute angles to compensate.