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 = a_2-a_1=1/12-1/15`
`=(5-4)/60 = 1/60`
We know that
`S_n = n/2[2a + (n -1)d]`
`S_11 = 11/2[2(1/15)+(11-1)1/60]`
`=11/2[2/5+10/60]`
`=11.2[2/15+1/6]=11/2[(4+5)/30]`
`=(11/2)(9/30)=33/20`
Concept: Sum of First n Terms of an AP
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