1. The problem statement, all variables and given/known data I have attached the question, along with the solution in the picture attached. This is one of the few questions I have encountered that I completely have no idea what the solution is trying to do... It's like they do not make any sense at all! Confused by 1. The summation on the right - i have no idea what is the purpose of it 2. The terms inside the brackets are also rather weird in terms of Mk-1 or Mk. The summation on the left - it starts from k = 2, 3, ..... which doesn't even match the terms in the brackets! 2. Relevant equations 3. The attempt at a solution I tried finding the d' Alembert's ratio but it gave me nonsensical answer of as long as x < 0 the series converges which is not true. If you compare the series to 1/n, the series diverges for x > -1 Will appreciate it if anyone can help.