mình cần gấp , ai lm dùm mình với ạ

Giải thích các bước giải:

a.Ta có: $AD//BC\to AD//BM\to \dfrac{ED}{EB}=\dfrac{EA}{EM}$

               $CD//AB\to DN//AB\to \dfrac{ED}{EB}=\dfrac{EN}{EA}$

$\to\dfrac{EA}{EM}=\dfrac{EN}{EA}$

$\to AE^2=EM\cdot EN$

b.Ta có:

$AE\cdot (\dfrac1{AM}+\dfrac1{AN})=\dfrac{AE}{AM}+\dfrac{AE}{AN}=\dfrac{DE}{DB}+\dfrac{BE}{BD}=\dfrac{DE+BE}{BD}=\dfrac{BD}{BD}=1$

$\to \dfrac1{AE}=\dfrac1{AM}+\dfrac1{AN}$

c.Ta có:

$\dfrac{AM}{AN}=\dfrac{BM}{BC}=\dfrac{BM}{AD}=\dfrac{BE}{DE}$

$\dfrac{FN}{EN}=\dfrac{NC}{ND}=\dfrac{CM}{AD}=\dfrac{CM}{BC}=\dfrac{FM}{FE}\to \dfrac{FM}{FN}=\dfrac{FE}{EN}=\dfrac{DC}{DN}=\dfrac{AB}{DN}=\dfrac{BE}{DE}$

$\to \dfrac{AM}{AN}=\dfrac{FM}{FN}$