9. Non-zero vectors $\vec{a}$ and $\vec{b}$ satisfy $|\vec{a}|=|\vec{b}|=|\vec{a}-\vec{b}|$, the angle between $\vec{a}$ and $\vec{a}+\vec{b}$ is
$\qquad$
9. $30^{\circ}$.

Construct a parallelogram with $\vec{a}$ and $\vec{b}$ as sides. Since $|\vec{a}|=|\vec{b}|=|\vec{a}-\vec{b}|$, it becomes a rhombus with one angle of $60^{\circ}$.