If the first term is a 1 and the tolerance is d, then (a1+d) (a1+3d) = 65,2a1+4d =18, and the simultaneous solution is a/kloc-0.

AI 2 = A 1 * A2 1, A 1 * A2 1 = 8 1, so ai=9, I = 3；;

According to arithmetic progression's summation formula, sn = 2n 2-n, if the formula in 3 is to be arithmetic progression, the root sign must be removed, sn+kn = 2n 2+(k-1) n, and k= 1 is known. At this time, the first term of the series is root number 2, the tolerance is root number 2, and the general term is.