Let a vertical tower A B of height 2 h stands on a horizontal ground. Let from a point P on the ground a man can see upto height h of the tower with…

Let a vertical tower $\displaystyle A B$ of height $\displaystyle 2 h$ stands on a horizontal ground. Let from a point $\displaystyle P$ on the ground a man can see upto height $\displaystyle h$ of the tower with an angle of elevation $\displaystyle 2 \alpha$. When from $\displaystyle P$, he moves a distance $\displaystyle d$ in the direction of $\displaystyle \overrightarrow{A P}$, he can see the top $\displaystyle B$ of the tower with an angle of elevation $\displaystyle \alpha$. If $\displaystyle d=\sqrt{7} h$, then $\displaystyle \tan \alpha$ is equal to