(1)连接BD、CD
因DG垂直且平分BC,所以：BD＝CD
AD平分∠BAC且DE⊥AB于E,DF⊥AC于F
DE＝DF
∠DEB=∠DFC=90°
△BDE≌△CDF
BE＝CF
(2)DE=DF,AD=AD,∠AED=∠AFD=90°
△AED≌△AFD,则：AE＝AF
AB－BE＝AC+CF＝AC+BE
BE＝(AB－AC)/2＝(a-b)/2
AE＝AB－BE＝a-(a-b)/2＝(a+b)/2