Pythagoras theorem is a rule that help us to find the unknown side of a rightangle triangle when two sides are known.We also use properties of parallel lines to solve for unknown angles.
Circle theorems are also rules or properties for finding unknown angles in plane geometry. When two or more plane shapes are drawn in a circle or from a point to a circle, these rules or properties help us to determine the missing angles.
Theorem 1
Angles in the same same segment
It is highly recommended that you perform the above activity before you proceed for good retention.
It can be inferred from activity 1 that:

angles created or subtended by a chord or arc in the same segment are equal

angles created or subtended by a chord or arc in the opposite segments sums up to 180^{0}.
Theorem 2
Angle about the centre
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It is highly recommended that you perform the above before you proceed for good retention.
It can be inferred from the above activity that:

the angle subtended by a chord or an arc at the centre is double the one at the centre created by the same chord.