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Question
Find the sum of the following
6 + 13 + 20 + .... + 97
Solution
6 + 13 + 20 + .... + 97
a = 6, d = 7, l = 97
n = `("l" - "a")/"d" + 1`
= `(97 - 6)/7 + 1 = 91/7 + 1`
= `(91 + 7)/7 = 98/7` = 14
Sn = `"n"/2 ("a" + "l")`
S14 = `14/2 (6 + 97)`
= 7 × 103
= 721
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