Mathematics for IGCSE & O level - Circle Theorems (Section 2 - No. 30)
Which of the following must be true in a circle?
Equal chords are equidistant from the center.
Chords that are equidistant from the center are equal in length.
The angle in a semicircle is a right angle.
Opposite angles of a cyclic quadrilateral are equal.
Explication
These are standard circle theorems. Only the last option is incorrect; they are supplementary not equal.