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Sequences and Series question

2018 · Shift 0 · Q33
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Sequences and Series question

2018 · Shift 0 · Q33

JEE MainMathematicsSequences and SeriesMCQ+4 / −1
Let a1{a_1}a1​, a2{a_2}a2​, a3{a_3}a3​, ......... , a49{a_{49}}a49​ be in A.P. such that ∑k=012a4k+1=416\sum\limits_{k = 0}^{12} {{a_{4k + 1}}} = 416k=0∑12​a4k+1​=416 and a9+a43=66{a_9} + {a_{43}} = 66a9​+a43​=66. a12+a22+.......+a172=140ma_1^2 + a_2^2 + ....... + a_{17}^2 = 140ma12​+a22​+.......+a172​=140m, then m is equal to
  1. A
    33
  2. B
    66
  3. C
    68
  4. D
    34
View written solutionFree

Correct answer: D

  1. Write the A.P. in standard form

Let the A.P. be an=a+(n−1)d,a_n=a+(n-1)d,an​=a+(n−1)d, where a=a1a=a_1a=a1​ and ddd is the common difference.


  1. Use the first condition

We are given ∑k=012a4k+1=416.\sum_{k=0}^{12} a_{4k+1}=416.∑k=012​a4k+1​=416.

Now, a4k+1=a+(4k)d.a_{4k+1}=a+(4k)d.a4k+1​=a+(4k)d. So the terms are: a1,a5,a9,…,a49.a_1,a_5,a_9,\dots,a_{49}.a1​,a5​,a9​,…,a49​. This is an A.P. with 13 terms.

Hence, ∑k=012a4k+1=132(a1+a49)=416.\sum_{k=0}^{12} a_{4k+1}=\frac{13}{2}(a_1+a_{49})=416.∑k=012​a4k+1​=213​(a1​+a49​)=416.

Now, a49=a+48d.a_{49}=a+48d.a49​=a+48d. So, 132(a+a+48d)=416\frac{13}{2}(a+a+48d)=416213​(a+a+48d)=416 132(2a+48d)=416\frac{13}{2}(2a+48d)=416213​(2a+48d)=416 13(a+24d)=41613(a+24d)=41613(a+24d)=416 a+24d=32.(1)a+24d=32. \qquad (1)a+24d=32.(1)


  1. Use the second condition

Given a9+a43=66.a_9+a_{43}=66.a9​+a43​=66.

Now, a9=a+8d,a43=a+42d.a_9=a+8d, \qquad a_{43}=a+42d.a9​=a+8d,a43​=a+42d. So, (a+8d)+(a+42d)=66(a+8d)+(a+42d)=66(a+8d)+(a+42d)=66 2a+50d=662a+50d=662a+50d=66 a+25d=33.(2)a+25d=33. \qquad (2)a+25d=33.(2)


  1. Solve for aaa and ddd

Subtract (1) from (2): (a+25d)−(a+24d)=33−32(a+25d)-(a+24d)=33-32(a+25d)−(a+24d)=33−32 d=1.d=1.d=1.

Put into (1): a+24=32a+24=32a+24=32 a=8.a=8.a=8.

Thus the A.P. is an=8+(n−1)=n+7.a_n=8+(n-1)=n+7.an​=8+(n−1)=n+7.


  1. Compute a12+a22+⋯+a172a_1^2+a_2^2+\cdots+a_{17}^2a12​+a22​+⋯+a172​

Since an=n+7a_n=n+7an​=n+7, a1,a2,…,a17=8,9,10,…,24.a_1,a_2,\dots,a_{17}=8,9,10,\dots,24.a1​,a2​,…,a17​=8,9,10,…,24.

Therefore, ∑n=117an2=82+92+⋯+242.\sum_{n=1}^{17} a_n^2=8^2+9^2+\cdots+24^2.∑n=117​an2​=82+92+⋯+242.

This is ∑r=124r2−∑r=17r2.\sum_{r=1}^{24} r^2-\sum_{r=1}^{7} r^2.∑r=124​r2−∑r=17​r2.

Using ∑r=1nr2=n(n+1)(2n+1)6,\sum_{r=1}^{n} r^2=\frac{n(n+1)(2n+1)}{6},∑r=1n​r2=6n(n+1)(2n+1)​, we get ∑r=124r2=24⋅25⋅496=4900,\sum_{r=1}^{24} r^2=\frac{24\cdot25\cdot49}{6}=4900,∑r=124​r2=624⋅25⋅49​=4900, ∑r=17r2=7⋅8⋅156=140.\sum_{r=1}^{7} r^2=\frac{7\cdot8\cdot15}{6}=140.∑r=17​r2=67⋅8⋅15​=140.

So, 82+92+⋯+242=4900−140=4760.8^2+9^2+\cdots+24^2=4900-140=4760.82+92+⋯+242=4900−140=4760.

Given a12+a22+⋯+a172=140m,a_1^2+a_2^2+\cdots+a_{17}^2=140m,a12​+a22​+⋯+a172​=140m, so 140m=4760140m=4760140m=4760 m=4760140=34.m=\frac{4760}{140}=34.m=1404760​=34.


  1. Check options

The value is m=34.m=34.m=34. So the correct option is D.


  1. Comparison with stored answer

Stored correct answer: D

Derived answer: D

They agree.

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