explain Part II only pls

1 Let us consider the series (n + 19)(n + 20) Note: Write the exact answer not the decimal approximation (for example write not 0.8). Answer. (1) Let { on } be the sequence of partial sums. Then Sn = 1 20 n+20 1 19 n+20 1 1 19 n+19 On = 1 1 20 n+19 (ii) If s is the sum of the series then