When n ≥ 6, an

An = 1 1-2n, when n ≥ 6, Tn = -Sn+2S5 (this step is very important, you must understand the meaning. That is, add each term to the inverse number (ensure that the latter term is positive), and of course, the first five terms have become negative. It doesn't matter, the sum of the first five items will be doubled) =-(10n-N2)+2 (10 * 5-52) = N2-10n+50.