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The three geometric means between 4 and 324 are

  • A.12,46,108
  • B.12,36,128
  • C.14,36,108
  • D.12,36,98
  • E.12,36,108

Answer: E

The geometric means are 12, 36, 108.

Inserting three geometric means between 4 and 324 makes a G.P. of five terms with first term 4 and fifth term 324.

\[ \begin{aligned} 4r^{4} &= 324 \\ r^{4} &= 81 \Rightarrow r = 3 \end{aligned} \]

So the terms are 4, 12, 36, 108, 324. Each of the other options has one wrong term: 46, 128, 14 and 98 do not continue a common ratio of 3.

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