Giải thích các bước giải:
a.Ta có : $AM//CD\to\dfrac{AM}{DC}=\dfrac{AI}{IC}$
$AD//CN\to\dfrac{AD}{CN}=\dfrac{AI}{IC}$
$\to \dfrac{AM}{DC}=\dfrac{AD}{CN}\to AM.CN=AD.DC=const$
b.Ta có : $AM//DC\to\dfrac{ID}{IM}=\dfrac{IC}{IA}$
$AD//CN\to\dfrac{ID}{IN}=\dfrac{IA}{IC}$
$\to \dfrac{ID}{IM}.\dfrac{ID}{IN}=\dfrac{IC}{IA}.\dfrac{IA}{IC}=1$
$\to ID^2=IM.IN$