So AB: BD = AC: DC = AB+AC: BD+DC = AB+AC: BC >1
So AB & gtBD, AC> DC
Modify the proportional formula
AB:AC=BD:DC
Subtract one from each side of the equation to get
(AB-AC):AC=(BD-DC):DC
(AB-AC):(BD-DC)=DC:AC
Because DC
So (ab-AC): (BD-DC) = DC: AC
So ab-AC