exterior angle property of a triangle

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.