AH ct EC ti F, nh vy tam gic AFC l tam gic cn v CH va l phn gic(t cu a ) va l chiu cao.Nh vy AH=HF, tam gic BHF=DHA(g.c.g) nn gc BFA=FAD do c KF song song AE V tam gic FHK=AHE(g.c.g) nn HK=HE, V FK=AE

Hai tam gic FKE V EAF C: KFE=FEA=VUNG( NH L KF song song VS AE), cnh FE chung, KF=AE do hai tam gic ny bng nhau(c.g.c) do KE=AF VY AH=HE=1/2 AF=1/2KE