opposite angles in a cyclic quadrilateral add up to 180°. A tangent is a straight line which touches the circle at a point but does not cut through the circle. There are two theorems involving ...

The second shape is not a cyclic quadrilateral. One vertex does not touch the circumference. The opposite angles in a cyclic quadrilateral add up to 180°.

The nine chapters deal respectively with preliminary ideas, trihedral angles, skew quadrilaterals ... but it is a useful collection of theorems on elementary topics. In particular, the chapters ...