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Sequences and Series question

2023 · 31 Jan · Shift 2 · Q29
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  5. /2023 · 31 Jan · Shift 2 · Q29

Sequences and Series question

2023 · 31 Jan · Shift 2 · Q29

JEE MainMathematicsSequences and SeriesMCQ+4 / −1
Let a1,a2,a3,…a_1, a_2, a_3, \ldotsa1​,a2​,a3​,… be an A.P. If a7=3a_7=3a7​=3, the product a1a4a_1 a_4a1​a4​ is minimum and the sum of its first nnn terms is zero, then n!−4an(n+2)n !-4 a_{n(n+2)}n!−4an(n+2)​ is equal to :
  1. A
    24
  2. B
    3814\frac{381}{4}4381​
  3. C
    9
  4. D
    334\frac{33}{4}433​
View written solutionFree

Correct answer: A

  1. Write the A.P. in standard form

Let the A.P. have first term aaa and common difference ddd. Then ar=a+(r−1)da_r=a+(r-1)dar​=a+(r−1)d

Given: a7=a+6d=3   (1)a_7=a+6d=3 \,\,\,\text{(1)}a7​=a+6d=3(1)

Also, a1a4=a(a+3d)a_1a_4=a(a+3d)a1​a4​=a(a+3d)

We are told this product is minimum.


  1. Use the condition for minimum product

From (1), a=3−6da=3-6da=3−6d

So, a1a4=a(a+3d)=(3−6d)(3−3d)a_1a_4=a(a+3d)=(3-6d)(3-3d)a1​a4​=a(a+3d)=(3−6d)(3−3d)

Expand: (3−6d)(3−3d)=9−27d+18d2(3-6d)(3-3d)=9-27d+18d^2(3−6d)(3−3d)=9−27d+18d2

This is a quadratic in ddd with positive coefficient, so its minimum occurs at d=−(−27)2⋅18=2736=34d=\frac{-(-27)}{2\cdot 18}=\frac{27}{36}=\frac34d=2⋅18−(−27)​=3627​=43​

Then a=3−6(34)=3−184=3−92=−32a=3-6\left(\frac34\right)=3-\frac{18}{4}=3-\frac92=-\frac32a=3−6(43​)=3−418​=3−29​=−23​

Hence the A.P. is a1=−32,d=34a_1=-\frac32,\qquad d=\frac34a1​=−23​,d=43​


  1. Use the condition that sum of first nnn terms is zero

Sum of first nnn terms of an A.P. is Sn=n2[2a+(n−1)d]S_n=\frac n2\left[2a+(n-1)d\right]Sn​=2n​[2a+(n−1)d]

Given Sn=0S_n=0Sn​=0. Since n≠0n\neq 0n=0, 2a+(n−1)d=02a+(n-1)d=02a+(n−1)d=0

Substitute a=−32a=-\frac32a=−23​ and d=34d=\frac34d=43​: 2(−32)+(n−1)34=02\left(-\frac32\right)+(n-1)\frac34=02(−23​)+(n−1)43​=0 −3+34(n−1)=0-3+\frac34(n-1)=0−3+43​(n−1)=0 34(n−1)=3\frac34(n-1)=343​(n−1)=3 n−1=4n-1=4n−1=4 n=5n=5n=5


  1. Find an(n+2)a_{n(n+2)}an(n+2)​

First compute: n(n+2)=5⋅7=35n(n+2)=5\cdot 7=35n(n+2)=5⋅7=35

So we need a35a_{35}a35​.

Using ar=a+(r−1)da_r=a+(r-1)dar​=a+(r−1)d a35=−32+34⋅34a_{35}=-\frac32+34\cdot \frac34a35​=−23​+34⋅43​ a35=−32+1024=−32+512=482=24a_{35}=-\frac32+\frac{102}{4}=-\frac32+\frac{51}{2}=\frac{48}{2}=24a35​=−23​+4102​=−23​+251​=248​=24

Thus, an(n+2)=24a_{n(n+2)}=24an(n+2)​=24


  1. Evaluate the required expression

The printed expression is n!−4an(n+2)n!-4a_{n(n+2)}n!−4an(n+2)​. With n=5n=5n=5, 5!−4a35=120−4(24)=120−96=245!-4a_{35}=120-4(24)=120-96=245!−4a35​=120−4(24)=120−96=24

So the value is 24\boxed{24}24​


  1. Check with options

Option A is 242424.

So the correct answer is A.

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