Giải thích các bước giải:
1.Ta có $OF\perp AD, OB\perp BD\to\Delta AOF\sim\Delta ADB(g.g)$
$\to\dfrac{AO}{AD}=\dfrac{AF}{AB}\to AO.AB=AF.AD$
2.Ta có $DB\perp AB, DO\perp BH, BK\perp DC$
$\to DB^2=DH.DO=DK.DC$
$\to \dfrac{DH}{DC}=\dfrac{DK}{DO}\to\Delta DHK\sim\Delta DCO(c.g.c)\to\widehat{DHK}=\widehat{DCO}$
$\to\Diamond KHOC$ nội tiếp
3.Ta có $OM\perp BC\to OM//BD$
$\to\widehat{MOD}=\widehat{ODB}=\widehat{ODM}\to MD=MO$
$\to\dfrac{BD}{DM}-\dfrac{DM}{AM}=\dfrac{BD}{OM}-\dfrac{DM}{AM}=\dfrac{AD}{AM}-\dfrac{DM}{AM}=\dfrac{AD-DM}{AM}=\dfrac{AM}{AM}=1$
