If $a, b, c$ are in AP, $b-a, c-b$ and $a$ are in GP, then $a: b: c$ is

If $a, b, c$ are in AP, $b-a, c-b$ and $a$ are in GP, then $a: b: c$ is
  1. $1: 2: 3$
  2. $1: 3: 5$
  3. $2: 3: 4$
  4. $1: 2: 4$

Solution

Given, that, $2 b=a+c$ and $(c-b)^2=(b-a) a$ $\begin{aligned} & \therefore \quad(b-a)^2=(b-a) a \\ & \Rightarrow \quad b=2 a \\ & \Rightarrow \quad c=3 a \\ & \Rightarrow \quad a: b: c=1: 2: 3 \\ & \end{aligned}$

Asked in: AP EAMCET 2007

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