1. Since cn ---> L, there must be a relationship between the elements of cn and the number L. State that relationship.
2. Now you need to show that the relationship stated in "1" indeed holds. To show this:
a. assume L = L1 + L2
b. use the given fact that the relationship stated in "1" holds between an and L1, as well as between bn and L2, to show that when L = L1 + L2, the relationship in "1" holds between cn and L.