Giải thích các bước giải:
a.Ta có :
$\dfrac{AE}{ED}=\dfrac{2}3\to \dfrac{BF}{FC}$
$\to \dfrac{AE}{AD}=\dfrac{2}5, \dfrac{CF}{CB}=\dfrac{3}5$
$\to \dfrac{EM}{DC}=\dfrac{2}5,\dfrac{FM}{AB}=\dfrac{3}5\to EM=12, FM=6$
$\to EF=EM+FM=18$
b.Ta có :
$AB//CD\to\dfrac{OA}{OC}=\dfrac{OB}{OD}\to OA.OD=OB.OC$
c.Ta có : $\dfrac{EN}{AB}=\dfrac{DE}{DA}=\dfrac{CF}{CB}=\dfrac{FM}{AB}\to EN=FM$
