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Sum

by shashank.ism » Mon Feb 08, 2010 7:16 am
If the sum of the first 11 terms of an arithmetic progression equals that of the first l9 terms, then what is the sum of the first 30 terms?

a) 0
b) -1
c) 1
d) 2
e) Not Unique
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by harsh.champ » Mon Feb 08, 2010 7:19 am
shashank.ism wrote:If the sum of the first 11 terms of an arithmetic progression equals that of the first l9 terms, then what is the sum of the first 30 terms?

a) 0
b) -1
c) 1
d) 2
e) Not Unique
a + (a+n)+............+(a+10n) = a + (a+n)+............+(a+18n)
=>11a + [10(10+1)/2]n = 19a + [18(18+1)/2]n
Solving this we get,
[spoiler]30a + 29(30)/2 = 0.

Hence,A. [/spoiler]
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