Let 10 vertical poles standing at equal distances on a straight line, subtend the same angle of elevation α at a point O on this line and all the…

Let $\displaystyle 10$ vertical poles standing at equal distances on a straight line, subtend the same angle of elevation $\displaystyle \alpha$ 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 :