7. Given that the three interior angles $\angle A, \angle B$, and $\angle C$ of $\triangle A B C$ form an arithmetic sequence, and the sides $A B=1, B C=4$. Then the length of the median $A D$ on side $B C$ is $\qquad$ .
7. $\sqrt{3}$.

It is known that $\angle B=60^{\circ}$. By the cosine rule, $A D=\sqrt{3}$.