Interactive resource
Triangle Angle Sum Visualizer
Geometry. Drag triangle vertices and verify that interior angles always sum to 180°. Free interactive geometry tool for class intro lessons.
Overview
No matter how you reshape a triangle, the interior angles sum to 180°. Dragging vertices lets learners confirm it themselves.
Explanation
In any plane triangle the three interior angles sum to 180°.
So two known angles determine the third; in a right triangle the acute angles are complementary.
Shape can change; the sum does not — a foundation for exterior angles and similarity.
How to use
- Open the triangle angle-sum scene.
- Drag the three vertices to change the shape.
- Watch the three angles and their total stay at 180°.
- Extend: given two angles, find the third.
Classroom example
Start acute, then pull into an obtuse triangle — angles change, the sum does not — before exterior angles and similarity.
Knowledge points
FAQ
Good for beginners?
Yes for intuition in early geometry; later courses can connect it to formal reasoning.
Does an obtuse triangle still sum to 180°?
Yes. Drag into an obtuse shape and the total stays 180° — verify live.
What should I open next?
Pythagoras or similar triangles — after angle sum, move to side relationships.
How do I find the third angle?
Third angle = 180° − (sum of the other two); check with this tool.