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Question: Answered & Verified by Expert
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 one of the following statements is correct?
MathematicsSequences and SeriesJEE AdvancedJEE Advanced 2007 (Paper 2)
Options:
  • A
    $G_1>G_2>G_3>\ldots$
  • B
    $G_1 < G_2 < G_3 < \ldots$
  • C
    $G_1=G_2=G_3=\ldots$
  • D
    $G_1 < G_3 < G_5 < \ldots$ and $G_2>G_4>G_6>\ldots$
Solution:
2794 Upvotes Verified Answer
The correct answer is:
$G_1=G_2=G_3=\ldots$
$A_1=\frac{a+b}{2}, G_1=\sqrt{a b}$ and $H_1=\frac{2 a b}{a+b}$ $\begin{aligned} A_n=\frac{A_{n-1}+H_{n-1}}{2}, G_n & =\sqrt{A_{n-1} H_{n-1}}, \\ H_n & =\frac{2 A_{n-1} H_{n-1}}{A_{n-1}+H_{n-1}}\end{aligned}$
Clearly, $\quad G_1=G_2=G_3=\ldots=\sqrt{a b}$.

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