Giải thích các bước giải:
a.Ta có $AD//CB$
$\to \dfrac{MA}{MC}=\dfrac{MD}{MK}$
Lại có $AB//CD$
$\to \dfrac{MA}{MC}=\dfrac{MN}{MD}$
$\to \dfrac{MD}{MK}=\dfrac{MN}{MD}$
$\to DM^2=MN.MK$
b.Ta có $AB//CD$
$\to \dfrac{DM}{DN}=\dfrac{CM}{CA}$
Lại có $AD//CK$
$\to \dfrac{DM}{DK}=\dfrac{AM}{AC}$
$\to \dfrac{DM}{DN}+\dfrac{DM}{DK}=\dfrac{CM}{CA}+\dfrac{AM}{AC}=\dfrac{CM+AM}{AC}=\dfrac{AC}{AC}=1$