Inscribed Angle and Central Angle
The pink angle is said to be an inscribed angle within the circle below.
This inscribed angle intercepts the thick blue arc of the circle.
Because of this, this thick blue arc is said to be the inscribed angle's intercepted arc.
Notice how the blue central angle also intercepts this same thick blue arc.
To start:
1) Move point D wherever you'd like.
2) Adjust the size of the thick blue intercepted arc by moving the other 2 blue points (if you wish.)
3) Click the checkbox to lock point D.
4) Follow the interactive prompts that will appear in the applet.