In triangle ABD and triangle ADC respectively.
AB2=BD2+AD2 ( 1)
AC2=AD2+CD2 (2)
( 1)- (2)
AB2-AC2 = BD2+AD2-AD2-CD2 = BD2-CD2 =(BD+CD)(BD-CD)(3)
BD+CD=BC (4)
According to (3)\(4), AB2-AC2=BC(BD-CD).