d) Cm : $\Delta BNM\sim \Delta BED\Rightarrow \frac{BN}{BE}=\frac{NM}{ED}$ (1)
* $CH.CO=CE^{2}=CA.CB\Rightarrow \Delta CAH\sim \Delta COB\Rightarrow AHOB$ nội tiếp.
$\Rightarrow \widehat{OHB}=\widehat{OAB}=\widehat{OBA}=\widehat{AHC}$
$\Rightarrow \widehat{AHE}=\widehat{BHE}=\frac{1}{2}\widehat{AHB}=\frac{1}2{}\widehat{AOB}$
Mà : $\widehat{ADB}=\frac{1}{2}\widehat{AOB}\Rightarrow \widehat{ADB}=\widehat{BHE}$
Mặt khác : $\widehat{ADB}=\widehat{BEN}=\widehat{BIN}$
$\Rightarrow \Delta BNI\sim \Delta BEH\Rightarrow \frac{BN}{BE}=\frac{NI}{EH}$ (2)
Từ (1)(2) $\Rightarrow \frac{NM}{ED}=\frac{NI}{EH}$ , mà $EH =\frac{1}{2}ED\Rightarrow NI=\frac{1}{2}NM\Rightarrow$ đpcm.