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Sequences and Series question

2023 · 13 Apr · Shift 1 · Q31
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  5. /2023 · 13 Apr · Shift 1 · Q31

Sequences and Series question

2023 · 13 Apr · Shift 1 · Q31

JEE MainMathematicsSequences and SeriesMCQ+4 / −1
Let s1,s2,s3,…,s10s_{1}, s_{2}, s_{3}, \ldots, s_{10}s1​,s2​,s3​,…,s10​ respectively be the sum to 12 terms of 10 A.P. s whose first terms are 1,2,3,….101,2,3, \ldots .101,2,3,….10 and the common differences are 1,3,5,……,191,3,5, \ldots \ldots, 191,3,5,……,19 respectively. Then ∑i=110si\sum_{i=1}^{10} s_{i}∑i=110​si​ is equal to :
  1. A
    7360
  2. B
    7220
  3. C
    7260
  4. D
    7380
View written solutionFree

Correct answer: C

  1. For the iii-th A.P.:

    • First term ai=ia_i = iai​=i, where i=1,2,…,10i=1,2,\dots,10i=1,2,…,10
    • Common difference di=2i−1d_i = 2i-1di​=2i−1 since the differences are 1,3,5,…,191,3,5,\dots,191,3,5,…,19
  2. Sum of first 121212 terms of an A.P. is si=122[2ai+(12−1)di]s_i=\frac{12}{2}\left[2a_i+(12-1)d_i\right]si​=212​[2ai​+(12−1)di​] si=6[2i+11(2i−1)]s_i=6\left[2i+11(2i-1)\right]si​=6[2i+11(2i−1)]

  3. Simplify: si=6[2i+22i−11]s_i=6\left[2i+22i-11\right]si​=6[2i+22i−11] si=6(24i−11)s_i=6(24i-11)si​=6(24i−11) si=144i−66s_i=144i-66si​=144i−66

  4. Now sum from i=1i=1i=1 to 101010: ∑i=110si=∑i=110(144i−66)\sum_{i=1}^{10} s_i = \sum_{i=1}^{10} (144i-66)∑i=110​si​=∑i=110​(144i−66) =144∑i=110i−66⋅10=144\sum_{i=1}^{10} i - 66\cdot 10=144∑i=110​i−66⋅10

  5. Use ∑i=110i=10⋅112=55\sum_{i=1}^{10} i = \frac{10\cdot 11}{2}=55∑i=110​i=210⋅11​=55 Therefore, ∑i=110si=144⋅55−660\sum_{i=1}^{10} s_i = 144\cdot 55 - 660∑i=110​si​=144⋅55−660 =7920−660=7920-660=7920−660 =7260=7260=7260

  6. Hence the correct option is 7260\boxed{7260}7260​ which is option C\boxed{\text{C}}C​.

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