Triangle Angles Anchor Chart
Angles & Triangles Guide
Geometry Reference • Angle Types, Classifications & Angle Sum Theorem
Golden Rule Total Interior = 180°
1
Types of Angles (Classified by Measure)
Acute Angle < 90°
45°
Between 0° and 90°
💡 "A cute little angle"
Right Angle = 90°
90°
Exactly 90°
💡 "Square corner box"
Obtuse Angle > 90°
130°
Between 90° and 180°
💡 "Open wide / big angle"
2
Types of Triangles (Classified by Angles)
Tip: Every triangle has 3 angles
Acute Triangle ALL 3 < 90°
60° 60° 60°
ALL 3 angles are acute (< 90°)
Example: 60° + 60° + 60° = 180°
Right Triangle ONE = 90°
90° 50° 40°
EXACTLY ONE right angle (90°)
Example: 90° + 50° + 40° = 180°
Obtuse Triangle ONE > 90°
120° 35° 25°
EXACTLY ONE obtuse angle (> 90°)
Example: 120° + 35° + 25° = 180°
3
Triangle Angle Sum Theorem
Angle A + Angle B + Angle C = 180°
The 180° Rule:
No matter what shape or size a triangle is, all 3 interior angles will always add up to 180° (a straight line)!
∠A ∠B ∠C Straight Line = 180°
How to Find a Missing Angle:
1
ADD the two known angles together.
2
SUBTRACT that sum from 180°.
Worked Example: Find the Missing Angle (?) Step-by-Step
? 70° 45° Given: 70° and 45°
Step 1: Add Known
70° + 45° = 115°
Combine the two angles
Step 2: Subtract from 180
180° - 115° = 65°
✓ Missing angle = 65°
Remember: • A triangle can only have ONE right or ONE obtuse angle. • The angles must always sum to 180°, never more, never less!
Math Anchor Chart