Hình thang ABCD có AB//CD à?
Vì [tex]OM//CD\Rightarrow \frac{OM}{DC}=\frac{OA}{OC}[/tex]
Vì [tex]ON//CD\Rightarrow \frac{OB}{OD}=\frac{ON}{CD}[/tex]
Vì [tex]AB//CD\Rightarrow \frac{OA}{OC}=\frac{OB}{OD}[/tex]
[tex]\Rightarrow \frac{OM}{DC}=\frac{ON}{CD}[/tex]
[tex]\Rightarrow OM=ON[/tex]