SOLUTION: 1.3 For an arithmetic series consisting of 15 terms Sn=2n-n^2 Determine: (a) the first term of the sequence. (b) the sum of the last 3 terms Please help me please

Question 1143233: 1.3 For an arithmetic series consisting of 15 terms Sn=2n-n^2
Determine:
(a) the first term of the sequence.
(b) the sum of the last 3 terms
Please help me please
Answer by greenestamps(13200) (Show Source): You can put this solution on YOUR website!

The n-th term is the difference between the sum of the first n terms and the sum of the first (n-1) terms:

So the first term is -2(1)+3 =1 and the common difference is -2.

ANSWERS:
(a) the first term is 1
(b) since the sequence is arithmetic, the sum of the last three terms (terms 13, 14, and 15) is 3 times the 14th term:

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