Depending on where you are, @XanderHenderson, or @ProfessorVector, for me, this is the evening before Monday due-dates and exams! Maybe I should hit the sack early tonight!!
Should the older question be closed as duplicate of the newer question? (The question posted today contains at least some attempt by the OP, so it is less likely to be closed because of missing context.)
@XanderHenderson Well, in that case we can ask mods to merge the questions. (Which moves the answer to the other one.) But let's see first whether it is going to be closed in this direction.
@XanderHenderson That may be not a good idea, because the system writes "Know someone who can answer?" under every unanswered question, too. Though I agee wholeheartedly: lots of people doing their exercises instead of asking others to do the job know how
There is one more near-duplicate - 1068566: Help with difficult telescoping series question. The only difference is that the sum ends at 2012. (Which probably is not that much of a difference.)