b) Áp dụng định lý $Thales$ ta có:
$+) \quad AN//CD$
$\to \dfrac{DM}{MN}= \dfrac{MC}{MA}$
$\to \dfrac{DM}{DN} = \dfrac{MC}{AC}$
$+)\quad AD//CK$
$\to \dfrac{DM}{MK} = \dfrac{MA}{MC}$
$\to \dfrac{DM}{DK} = \dfrac{MA}{AC}$
Cộng vế theo vế ta được:
$\dfrac{DM}{DN} + \dfrac{DM}{DK} = \dfrac{MC}{AC} + \dfrac{MA}{AC} = 1$