Let 10 vertical poles standing at equal distances on a straight...

Let $10$ vertical poles standing at equal distances on a straight line, subtend the same angle of elevation $\alpha$ at a point $O$ on this line and all the poles are on the same side of $O$. If the height of the longest pole is $h$ and the distance of the foot of the smallest pole from $O$ is $a$; then the distance between two consecutive poles, is