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Sequences and Series question

2020 · 2 Sep · Shift 2 · Q29
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Sequences and Series question

2020 · 2 Sep · Shift 2 · Q29

JEE MainMathematicsSequences and SeriesMCQ+4 / −1
If the sum of first 11 terms of an A.P., a1, a2, a3, .... is 0 (a eee 0), then the sum of the A.P., a1 , a3 , a5 ,....., a23 is ka1 , where k is equal to :
  1. A
    12110{{121} \over {10}}10121​
  2. B
    -12110{{121} \over {10}}10121​
  3. C
    725{{72} \over 5}572​
  4. D
    -725{{72} \over 5}572​
View written solutionFree

Correct answer: D

  1. Let the A.P. be a1, a2, a3,…a_1,\, a_2,\, a_3,\dotsa1​,a2​,a3​,… with first term a1a_1a1​ and common difference ddd.

  2. Given that the sum of the first 111111 terms is 000: S11=112[2a1+10d]=0S_{11} = \frac{11}{2}\left[2a_1 + 10d\right] = 0S11​=211​[2a1​+10d]=0 Since 11≠011 \neq 011=0, 2a1+10d=02a_1 + 10d = 02a1​+10d=0 a1+5d=0a_1 + 5d = 0a1​+5d=0 d=−a15d = -\frac{a_1}{5}d=−5a1​​

  3. Now consider the sequence a1, a3, a5,…,a23a_1,\, a_3,\, a_5,\dots, a_{23}a1​,a3​,a5​,…,a23​ These are the odd-numbered terms of the original A.P.

  4. Number of terms from a1a_1a1​ to a23a_{23}a23​ taking only odd indices: 1,3,5,…,231,3,5,\dots,231,3,5,…,23 This is an A.P. in indices with common difference 222. Number of terms: n=23−12+1=12n = \frac{23-1}{2}+1 = 12n=223−1​+1=12

  5. The odd-indexed terms also form an A.P.:

    • First term =a1= a_1=a1​
    • Common difference =2d= 2d=2d

    So their sum is 122[2a1+(12−1)(2d)]\frac{12}{2}\left[2a_1 + (12-1)(2d)\right]212​[2a1​+(12−1)(2d)] =6[2a1+22d]= 6\left[2a_1 + 22d\right]=6[2a1​+22d]

  6. Substitute d=−a15d = -\frac{a_1}{5}d=−5a1​​: 6[2a1+22(−a15)]6\left[2a_1 + 22\left(-\frac{a_1}{5}\right)\right]6[2a1​+22(−5a1​​)] =6[2a1−22a15]= 6\left[2a_1 - \frac{22a_1}{5}\right]=6[2a1​−522a1​​] =6[10a1−22a15]= 6\left[\frac{10a_1 - 22a_1}{5}\right]=6[510a1​−22a1​​] =6(−12a15)= 6\left(-\frac{12a_1}{5}\right)=6(−512a1​​) =−725a1= -\frac{72}{5}a_1=−572​a1​

  7. Hence, a1+a3+a5+⋯+a23=ka1a_1 + a_3 + a_5 + \cdots + a_{23} = k a_1a1​+a3​+a5​+⋯+a23​=ka1​ gives k=−725k = -\frac{72}{5}k=−572​

  8. Therefore, the correct option is D\boxed{\text{D}}D​

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