Đáp án:
a, Xet tam giac BDE co
AM//DE => AM/DE = BM/BD ==>> DE/AM=BD/BM
b, co AM/DF= CM/CD ( vi tam giac AMC co FD//AM)
==>> AM/CM= DF/CD
co AM/DE =BM/BD
===>> AM/BM =DE/DB
ma CM=BM ==> AM/CM=AM/BM= AM+AM/ CM+BM= 2AM/BC (1)
==>> DF/CD= DE/DB = DE+DF/ CD+BD = DE+DF/BC (2)
TU (1) va (2) ==>> DE+DF= 2AM
c, co EY/YD = AE/AB ==>EY/YD= MD/BM =.> EY/YD= AY/ BM ( MD=AY vi AYDM la HBH )
==> EY/AY = YD/BM = AM/BM
co YF/AY = FD/DC = AM/CM ma CM=BM ==>> EY=YF => Y la trung diem EF :)))))
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