Inscribed angleEither angle given
One angle is given. Find the one marked with a question mark.
Two angles standing on the same arc: one at the center of the circle, one out on the rim. The one at the center is always twice the other. These free printable worksheets ask for each direction, and mix them.
If they get stuck

The answer key prints on a separate page.
This is a theorem rather than a formula, and the difference shows in what a child has to do. There is nothing to measure and nothing to substitute — the whole question is reading which of the two marked angles was given, and then halving or doubling. Which is exactly why a page of one direction is not enough practice: handed the center angle the mistake is doubling, handed the rim angle it is halving, and the mixed page is the one where the figure has to be read first.
Both angles are marked on every figure, one with its size and one with a question mark, because the theorem is a statement about a pair. A figure showing one angle would be showing half the question, and the two arcs are drawn at different radii so a 30° angle and a 150° one do not read the same at a glance.
One question from a sheet like this one, worked a step at a time — with a sentence saying what just happened and why.
Inscribed angle in a circle, step by step