JEE Advanced · Mathematics · 5. Sequences & Series
Paragraph:
Let \(A_1, G_1, H_1\) denote the arithmetic, geometric and harmonic means respectively, of two distinct positive numbers. For \(n \geq 2\), let \(A_{n-1}\) and \(H_{n-1}\) has arithmetic, geometric and harmonic means as \(A_n, G_n, H_n\), respectively.
Question:
Which of the following statements is correct?
- A \(A_1>A_2>\ldots\)
- B \(A_1 < A_2 < A_3 < \ldots\)
- C \(A_1>A_3>A_5>\ldots\) and \(A_2 < A_4 < A_6 < \ldots\)
- D \(A_1 < A_3 < A_5 < \ldots\) and \(A_2>A_4>A_6>\ldots\)
Answer & Solution
Correct Answer
(A) \(A_1>A_2>\ldots\)
Step-by-step Solution
Detailed explanation
Let \(f(x)=k e^x-x\)
\(f^{\prime}(x) =k e^x-1=0 \)
\( x =-\ln k \)
\( \therefore f^{\prime \prime}(x) =k e^x \)
\( \text {Hence, } {\left[f^{\prime \prime}(x)\right]_{x=-\ln k} } =1>0 \)
\( f(-\ln k) =1+\ln k\)
For one root of given equation
\(1+\ln k =0\)
Hence, \(k =\frac{1}{e}\)
\(A_2\) is \(A M\) of \(A_1\) and \(H_1\) and \(A_1>H_1\)
\(
\Rightarrow A_1>A_2>H_1
\)
\(A_3\) is \(\mathrm{AM}\) of \(\mathrm{A}_2\) and \(\mathrm{H}_2\) and \(\mathrm{A}_2>\mathrm{H}_2\)
\(\Rightarrow A_2>A_3>H_2 \)
\( \therefore A_1>A_2>A_3>\ldots\)
\(f^{\prime}(x) =k e^x-1=0 \)
\( x =-\ln k \)
\( \therefore f^{\prime \prime}(x) =k e^x \)
\( \text {Hence, } {\left[f^{\prime \prime}(x)\right]_{x=-\ln k} } =1>0 \)
\( f(-\ln k) =1+\ln k\)
For one root of given equation
\(1+\ln k =0\)
Hence, \(k =\frac{1}{e}\)
\(A_2\) is \(A M\) of \(A_1\) and \(H_1\) and \(A_1>H_1\)
\(
\Rightarrow A_1>A_2>H_1
\)
\(A_3\) is \(\mathrm{AM}\) of \(\mathrm{A}_2\) and \(\mathrm{H}_2\) and \(\mathrm{A}_2>\mathrm{H}_2\)
\(\Rightarrow A_2>A_3>H_2 \)
\( \therefore A_1>A_2>A_3>\ldots\)
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