# exterior angle property of a triangle

- Author:
- Mathguru

- Topic:
- Angles

We can see a triangle ABC in blue color. We can also see that a line CD has been made at an angle У to line BC. Angle BAC = α, angle ABC = β and angle BCD = У. Points A, B, and C are free points. We can change the value of У using its slide bar. Move the three points A, B and C and change the value of У using the slide bar and see how the shape of the triangle and angles gets changed.

Questions to think about
1. Move the points A, B and C and calculate the value of (α+β). Then set this sum on the slide bar, what do we observe?