Progression (Sequence and Series)HardBloom L3

Question

If between any two quantities there be inserted two arithmetic means $A_{1},{\text{ }A}_{2}$; two geometric means $G_{1},G_{2}$; and two harmonic means $H_{1},H_{2}$, then $G_{1}G_{2}:A_{1} + A_{2}$ is equal to

Options

A.$H_{1}H_{2}:H_{1} + H_{2}$
B.$\frac{1}{H_{1}} + \frac{1}{H_{2}}$
C.$H_{1} + H_{2}$
D.$H_{1}H_{2}$

Solution

Sol. [Key Concept: Insert 2 A.M.s, 2 G.M.s, 2 H.M.s between two numbers and combine the standard mean relations.]

$${\frac{1}{H_{1}} + \frac{1}{H_{2}} = \frac{1}{a} + \frac{1}{\text{ }b} = \frac{a + b}{ab} = \frac{A_{1} + A_{2}}{G_{1}G_{2}} }{\Rightarrow \frac{H_{1} + H_{2}}{H_{1}H_{2}} = \frac{A_{1} + A_{2}}{G_{1}G_{2}}}$$

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