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Sequences and Series question

2024 · 29 Jan · Shift 2 · Q41
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Sequences and Series question

2024 · 29 Jan · Shift 2 · Q41

JEE MainMathematicsSequences and SeriesMCQ+4 / −1
If log⁡ea,log⁡e b,log⁡ec\log _e \mathrm{a}, \log _e \mathrm{~b}, \log _e \mathrm{c}loge​a,loge​ b,loge​c are in an A.P. and log⁡ea−log⁡e2 b,log⁡e2 b−log⁡e3c,log⁡e3c−log⁡e\log _e \mathrm{a}-\log _e 2 \mathrm{~b}, \log _e 2 \mathrm{~b}-\log _e 3 \mathrm{c}, \log _e 3 \mathrm{c} -\log _eloge​a−loge​2 b,loge​2 b−loge​3c,loge​3c−loge​ a are also in an A.P, then a:b:ca: b: ca:b:c is equal to
  1. A
    6:3:26: 3: 26:3:2
  2. B
    9:6:49: 6: 49:6:4
  3. C
    25:10:425: 10: 425:10:4
  4. D
    16:4:116: 4: 116:4:1
View written solutionFree

Correct answer: B

  1. Since log⁡ea,log⁡eb,log⁡ec\log_e a,\log_e b,\log_e cloge​a,loge​b,loge​c are in A.P., the middle term is the average of the other two:

2log⁡b=log⁡a+log⁡c2\log b = \log a + \log c2logb=loga+logc

Using log properties,

log⁡b2=log⁡(ac)  ⟹  b2=ac...(1)\log b^2 = \log(ac) \implies b^2 = ac \quad ...(1)logb2=log(ac)⟹b2=ac...(1)

  1. Now consider the second A.P.:

log⁡a−log⁡2b,log⁡2b−log⁡3c,log⁡3c−log⁡a\log a - \log 2b,\quad \log 2b - \log 3c,\quad \log 3c - \log aloga−log2b,log2b−log3c,log3c−loga

First simplify each term:

log⁡a−log⁡2b=log⁡(a2b)\log a - \log 2b = \log\left(\frac{a}{2b}\right)loga−log2b=log(2ba​) log⁡2b−log⁡3c=log⁡(2b3c)\log 2b - \log 3c = \log\left(\frac{2b}{3c}\right)log2b−log3c=log(3c2b​) log⁡3c−log⁡a=log⁡(3ca)\log 3c - \log a = \log\left(\frac{3c}{a}\right)log3c−loga=log(a3c​)

Since these are in A.P.,

2log⁡(2b3c)=log⁡(a2b)+log⁡(3ca)2\log\left(\frac{2b}{3c}\right)=\log\left(\frac{a}{2b}\right)+\log\left(\frac{3c}{a}\right)2log(3c2b​)=log(2ba​)+log(a3c​)

Combine logs:

log⁡(4b29c2)=log⁡(a2b⋅3ca)\log\left(\frac{4b^2}{9c^2}\right)=\log\left(\frac{a}{2b}\cdot \frac{3c}{a}\right)log(9c24b2​)=log(2ba​⋅a3c​)

log⁡(4b29c2)=log⁡(3c2b)\log\left(\frac{4b^2}{9c^2}\right)=\log\left(\frac{3c}{2b}\right)log(9c24b2​)=log(2b3c​)

Hence,

4b29c2=3c2b\frac{4b^2}{9c^2}=\frac{3c}{2b}9c24b2​=2b3c​

Cross-multiplying,

8b3=27c38b^3 = 27c^38b3=27c3

(bc)3=(32)3\left(\frac{b}{c}\right)^3 = \left(\frac{3}{2}\right)^3(cb​)3=(23​)3

So,

bc=32...(2)\frac{b}{c} = \frac{3}{2} \quad ...(2)cb​=23​...(2)

  1. Using (1)(1)(1), b2=acb^2 = acb2=ac, so

a=b2ca = \frac{b^2}{c}a=cb2​

From b:c=3:2b:c = 3:2b:c=3:2, let

b=3k,c=2kb=3k,\quad c=2kb=3k,c=2k

Then

a=(3k)22k=9k2a = \frac{(3k)^2}{2k} = \frac{9k}{2}a=2k(3k)2​=29k​

Thus,

a:b:c=9k2:3k:2ka:b:c = \frac{9k}{2}:3k:2ka:b:c=29k​:3k:2k

Multiply by 222:

a:b:c=9:6:4a:b:c = 9:6:4a:b:c=9:6:4

  1. Therefore the correct option is:

B  (9:6:4)\boxed{B\; (9:6:4)}B(9:6:4)​

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