Question

Let ABCD be a cyclic quadrilateral with sides AB = 2 cm, BC = 7 cm, CD = 11 cm, DA = 14 cm. Find the product of its diagonals.

Answer 1

Using Ptolemy's theorem,
$AC\times BD=(AB\times CD)+(AD\times BC)$
Substituting the values,
AC × BD = (2 × 11) + (14 × 7)
=120 cm