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Question
Find the sum of last ten terms of the AP: 8, 10, 12,.., 126.
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Solution
For finding the sum of last ten terms, we write the given AP in reverse order.
i.e., 126, 124, 122,..., 12, 10, 8
Here, first term (a) = 126,
Common difference (d) = 124 – 126 = –2
∴ S10 = `10/2 [2a + (10 - 1)d]` ...`[∵ S_n = n/2[2a + (n - 1)d]]`
= 5{2(126) + 9(–2)}
= 5(252 – 18)
= 5 × 234
= 1170
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