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Sequences and Series question

2025 · 3 Apr · Shift 1 · Q27
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Sequences and Series question

2025 · 3 Apr · Shift 1 · Q27

JEE MainMathematicsSequences and SeriesMCQ+4 / −1
Let a1,a2,a3,…a_1, a_2, a_3, \ldotsa1​,a2​,a3​,…. be a G.P. of increasing positive numbers. If a3a5=729a_3 a_5=729a3​a5​=729 and a2+a4=1114a_2+a_4=\frac{111}{4}a2​+a4​=4111​, then 24(a1+a2+a3)24\left(a_1+a_2+a_3\right)24(a1​+a2​+a3​) is equal to
  1. A
    128
  2. B
    129
  3. C
    131
  4. D
    130
View written solutionFree

Correct answer: B

  1. Write the G.P. terms in standard form

Let the G.P. be a1=a, a2=ar, a3=ar2, a4=ar3, a5=ar4a_1=a,\, a_2=ar,\, a_3=ar^2,\, a_4=ar^3,\, a_5=ar^4a1​=a,a2​=ar,a3​=ar2,a4​=ar3,a5​=ar4 with a>0a>0a>0 and r>1r>1r>1 since the terms are increasing positive numbers.

  1. Use the condition a3a5=729a_3a_5=729a3​a5​=729

We have a3a5=(ar2)(ar4)=a2r6=(ar3)2=729a_3a_5=(ar^2)(ar^4)=a^2r^6=(ar^3)^2=729a3​a5​=(ar2)(ar4)=a2r6=(ar3)2=729 So, ar3=27ar^3=27ar3=27

  1. Use the condition a2+a4=1114a_2+a_4=\frac{111}{4}a2​+a4​=4111​

ar+ar3=a(r+r3)=1114ar+ar^3=a(r+r^3)=\frac{111}{4}ar+ar3=a(r+r3)=4111​ Factor: ar(1+r2)=1114ar(1+r^2)=\frac{111}{4}ar(1+r2)=4111​ Since ar3=27ar^3=27ar3=27, we get ar=27r2ar=\frac{27}{r^2}ar=r227​ So 27r2(1+r2)=1114\frac{27}{r^2}(1+r^2)=\frac{111}{4}r227​(1+r2)=4111​ 27(1+r2r2)=111427\left(\frac{1+r^2}{r^2}\right)=\frac{111}{4}27(r21+r2​)=4111​ 27(1+1r2)=111427\left(1+\frac{1}{r^2}\right)=\frac{111}{4}27(1+r21​)=4111​ Multiply by 444: 108(1+1r2)=111108\left(1+\frac{1}{r^2}\right)=111108(1+r21​)=111 108+108r2=111108+\frac{108}{r^2}=111108+r2108​=111 108r2=3\frac{108}{r^2}=3r2108​=3 r2=36r^2=36r2=36 Since r>1r>1r>1, r=6r=6r=6

  1. Find aaa

From ar3=27ar^3=27ar3=27 we get a⋅63=27a\cdot 6^3=27a⋅63=27 216a=27216a=27216a=27 a=18a=\frac{1}{8}a=81​

Thus, a1=18,a2=68=34,a3=368=92a_1=\frac18,\quad a_2=\frac68=\frac34,\quad a_3=\frac{36}{8}=\frac92a1​=81​,a2​=86​=43​,a3​=836​=29​

  1. Compute 24(a1+a2+a3)24(a_1+a_2+a_3)24(a1​+a2​+a3​)

a1+a2+a3=18+34+92a_1+a_2+a_3=\frac18+\frac34+\frac92a1​+a2​+a3​=81​+43​+29​ Convert to denominator 888: =18+68+368=438=\frac18+\frac68+\frac{36}{8}=\frac{43}{8}=81​+86​+836​=843​ Therefore, 24(a1+a2+a3)=24⋅438=3⋅43=12924(a_1+a_2+a_3)=24\cdot \frac{43}{8}=3\cdot 43=12924(a1​+a2​+a3​)=24⋅843​=3⋅43=129

  1. Check options

The value is 129\boxed{129}129​ So the correct option is B.

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