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Question
Find the sum of the following APs:
`1/15, 1/12, 1/10`, ... to 11 terms.
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Solution
`1/15, 1/12, 1/10,` ... to 11 terms
For this A.P.,
a = `1/15`
n = 11
d = a2 – a1
= `1/12 - 1/15`
= `(5 - 4)/60`
= `1/60`
We know that
Sn = `n/2[2a + (n - 1)d]`
S11 = `11/2[2(1/15) + (11 - 1)1/60]`
S11 = `11/2[2/5 + 10/60]`
S11 = `11.2[2/15 + 1/6]`
S11 = `11/2[(4 + 5)/30]`
S11 = `(11/2)(9/30)`
S11 = `33/20`
Thus, the required sum of first 11 terms is `33/20`.
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