JEE MainMathematicsSequences and SeriesMCQ+4 / −1
If the sum of first 11 terms of an A.P., a1, a2, a3, .... is 0 (a 0), then the sum of the A.P., a1 , a3 , a5 ,....., a23 is ka1 , where k is equal to :
- A
- B-
- C
- D-
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Correct answer: D
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Let the A.P. be with first term and common difference .
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Given that the sum of the first terms is : Since ,
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Now consider the sequence These are the odd-numbered terms of the original A.P.
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Number of terms from to taking only odd indices: This is an A.P. in indices with common difference . Number of terms:
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The odd-indexed terms also form an A.P.:
- First term
- Common difference
So their sum is
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Substitute :
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Hence, gives
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Therefore, the correct option is
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