Shouldn’t 240 also be considered since it’s close to 239? It makes sense to exclude 239, but 240 seems like a valid choice too. Is there a quicker method to solve this? Using the quadratic formula and finding roots is quite time-consuming.
Also, the solution links are incorrect; they lead only to the topic video instead of the specific solution. Could you please share where can solutions of these be found?
I mean 240 and practically everything could work, and in that sense it may be a bit ambiguous. But like at one point, you should try to understand unstated context/assumptions and not be nitpicky about it ig. The convention is that n should be a part of the series and 240 is not a part of the sequence a_n = 4n - 1. In other words, if n = 240 was possible then you’re admitting that the sum isn’t an arithmetic series and it’s something else altogether.
39, 40, and 41 are too small by inspection so those are definitely not right. 240 is a multiple of 4 so that doesn’t work either, which leaves you with only two choices to check. Using the fact that the sum is monotonically increasing, you only have to check one of the two choices. For example, if you check 239 and u see that the left hand side evaluates exactly to 7260 then 279 is the only candidate and vice versa.
This makes sense. Thank you. The method you suggested is definitely a time saver.
Hey @Leaderboard, could you please upload or attach the solutions to the Sequence and Series exercises on Prep Swift? Apologies for tagging you like this, but my GRE is in just 2 days, and I wanted to review the actual solutions to the quizzes to ensure my approaches are correct.
Thank you!
Most of the problems have solutions (not this one). I’ll try to get solutions out soon, but please ask here if there are questions that do not have a solution.
