∫ad is the bisector of ∠BAC, ∴∠DAE=∠DAC,

Because AD is male marginal,

∴△ADE≌△ADC,(S,A,S)，

∴DE=CD，

In the triangular bed, BE=AB-AE=AB-AC,

BD-DC=BD-DE, where be > BD-de,

∴ ab-AC > BD-DC (in a triangle, the difference between two sides is smaller than the third side).