10. In the following diagram of a trapezium, $OB$ is parallel to $AC$, and $BD:DA=1:2$. Given that $\overrightarrow{OA} = 4\mathbf{a}$, $\overrightarrow{OB} = 3\mathbf{b}$ and $\overrightarrow{AC} = 6\mathbf{b}$, show that $ODC$ is a straight line and find the ratio $OD:DC$.
$\overrightarrow{OC} = 4\mathbf{a} + 6\mathbf{b}$, $\overrightarrow{OD} = \frac{1}{3}\overrightarrow{OC}$.
Ratio $1:2$, parallel and share a point.