25. (a) The sum of the first n terms of a sequence is given by S, = n(n+2).
Find:
(1) the sum of the twentieth,
twenty-first and twenty-second
terms of the sequence;
(ii) the first three terms of the
sequence.
(b) The sum of the first twenty-one
terms of a linear sequence (A. P.) is
28, and the sum of the first twenty-
eight terms is 21. Find which term of
the sequence is 0 and also the sum
of the terms preceding it.
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