*Let ABCD be a cyclic quadrilateral with lengths of sides AB = p , BC = q , CD = r, and DA = s . Show that \(\frac{AC}{BD} = \frac{ps+qr}{pq+rs} \)*

**Discussion:**

Since ABCD is cyclic \(\Delta ABE \) is similar to \(\Delta CDE \) (since \(\angle ABE = \angle DCE \) as they are subtended by the same arc AD, and \(\angle AEB = \angle CED \) as vertically opposite angles are equal)

Hence their corresponding sides are proportional.

\(\frac{p}{r} = \frac{BE}{CE} \implies BE = CE \times \frac{p}{r} \)

\(\frac{p}{r} = \frac{AE}{DE} \implies AE = DE \times \frac{p}{r} \)

Similarly \(\Delta AED \) is similar to \(\Delta BEC \)

\(\frac{s}{q} = \frac{DE}{CE} \implies DE = CE \times \frac{s}{q} \)

\(\frac{q}{s} = \frac{CE}{DE} \implies CE = DE \times \frac{q}{s} \)

Hence

\(BE + DE = BD = CE \times { \frac{p}{r} + \frac{s}{q} } = CE \times \frac{pq+rs}{qr} \)

\(AE + CE = AC = DE \times { \frac{p}{r} + \frac{q}{s} } = DE \times \frac{ps+qr}{sr} \)

Hence \(\displaystyle { \frac{AC}{BD} = \frac {ps + qr}{pq + rs} \times \frac {qr}{sr} \times \frac {DE}{CE} }\)

Finally we note that since \(\Delta AED \) is similar to \(\Delta BEC \). \(\displaystyle {\frac{q}{s} = \frac {CE}{DE} \implies \frac{q}{s} \times \frac {DE}{CE} = 1 } \)

This proves that

\(\displaystyle { \frac{AC}{BD} = \frac {ps + qr}{pq + rs}}\)