Ta có:
$\frac{MD}{MC}=\frac{AD}{BC}$
$\Rightarrow \frac{MD}{AD}=\frac{MC}{BC}$
$\Rightarrow \frac{MD}{AD}=\frac{MC}{CK}$
$\Rightarrow \frac{MD}{AD}=\frac{MC+CK}{CK}$
$\Rightarrow \frac{MD}{AD}=\frac{MK}{CK}$
$\Rightarrow \frac{MD}{MK}=\frac{AD}{CK}$
Vậy KM là tia phân giác của CKD.