7. Given the circle $x^{2}+y^{2}=1$ and the line $y=2 x+m$ intersect at $A$ and $B$, and $O A$ and $O B$ form angles $\alpha, \beta$ with the positive direction of the $x$-axis, then $\sin (\alpha+\beta)=$ $\qquad$
$-\frac{4}{5} ;$