The third sub problem of the key point is known that in the sequence {an}, A1 = 1, Nan + 1 = 2 (a1 + A2 + a3 +... + an) (n ∈ n *) (1) the general formula for finding A2, A3, A4 (2) for finding an (3) Let {BN} satisfy B1 = 1 / 2, B (n + 1) = 1 / AK * BN ^ 2 + BN, and prove BN

The third sub problem of the key point is known that in the sequence {an}, A1 = 1, Nan + 1 = 2 (a1 + A2 + a3 +... + an) (n ∈ n *) (1) the general formula for finding A2, A3, A4 (2) for finding an (3) Let {BN} satisfy B1 = 1 / 2, B (n + 1) = 1 / AK * BN ^ 2 + BN, and prove BN

Let me have a try... (1) from the problem, Nan + 1 = 2Sn, A1 = 1A2 = 2S1 = 2A1 = 2A3 = 1 / 2 * 2s2 = S2 = a1 + A2 = 3A4 = 1 / 3 * 2s3 = 2 / 3 [A1 + A2 + A3] = 4 (2) Nan + 1 = n (Sn + 1 - Sn) = 2snsn + 1 / Sn = (n + 2) / NSN / sn-1 = (n + 1) / (n-1) (n ≥ 2), Sn / S1 = Sn / A1 = Sn = (n + 1) / (n-1) / (n-2)