ABC is a right-angled triangle in which angle B=90 degree and BC =a. If n points L1,L2,L3,...,Ln on AB is divided in n+1 equal parts and l1M1,L2M2,...,LnMn are line segments parallel to BC and M1,M2,...Mn are on AC then find find the sum of the lenghts of L1M1,L2M2,...LnMn.