Find a missing triangle angle from the two angles you know.
This is the first lesson in its subject. A free account reads it in full.
You are reading the opening. Members read every lesson and guide in full. A free account reads the first lesson in each subject in full.
You are reading the opening. Your free account reads the first lesson in each subject in full. Members read every lesson and guide in full.
We couldn’t check your membership. Please reload and try again.
Members read every lesson and guide in full and practice without limits. View membership
The three interior angles of any triangle on a flat plane total 180°. Changing the side lengths changes individual angles, but not their sum.
In the first triangle, 40° + 60° + 80° = 180°. Use the stated measurements. A drawing can suggest a relationship, but its apparent size is not evidence.
This is the first lesson in its subject. A free account reads it in full.
We couldn’t check your membership. Please reload and try again.
Members read every lesson and guide in full and practice without limits. View membership
Practice · Use triangle sums, parallel lines and exterior angles. Move the diagram to explore the relationships.
Book · Explore measurements, transformations, coordinates and geometric relationships through diagrams and worked examples.
Next lesson · Measure perpendicular height to compare triangle areas without relying on shape.
Browser storage is unavailable. Changes last only on this page.