3. In Fig, $\mathrm{ABC}$ and $\mathrm{DBC}$ are two triangles on the same base $\mathrm{BC}$. If AD intersects $\mathrm{BC}$ at $\mathrm{O}$, show that
$\frac{\operatorname{ar}(\mathrm{ABC})}{\operatorname{ar}(\mathrm{DBC})}=\frac{\mathrm{AO}}{\mathrm{DO}}$