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Question
Find the sum of first 20 terms of an A.P. whose nth term is given as an = 5 – 2n.
Solution
We have, an = 5 – 2n
∴ a1 = 5 – 2 = 3
and a2 = 5 – 4 = 1
∴ d(common difference) = 1 – 3 = –2
Now, S20 = `20/2[2 xx 3 + (20 - 1)(-2)]` ......`[∵ S_n = n/2[2a + (n - 1)d]]`
= 10[6 – 19 × 2]
= 10[6 – 38]
= 10 × (–32)
= –320
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