12. In equilateral triangle $ABC$, $D$ is a point on side $BC$. If $AB = 3, BD = 1$, then $\overrightarrow{AB} \cdot \overrightarrow{AD} =$ $\_\_\_\_$
12. In equilateral triangle $ABC$, $D$ is a point on side $BC$. If $AB = 3, BD = 1$, then $\overrightarrow{AB} \cdot \overrightarrow{AD} =$ $\_\_\_\_$\\