NDA 2019 Maths · Mathematics · Sequences and Series · easy

If two geometric means are inserted between 3 and 81, their sum is

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If two geometric means are inserted between

3

and

81

, their sum is

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Answer: (D) $36$

Four terms

3,\,3r,\,3r^2,\,81

are in GP, so

3r^3=81

and

r=3

. The means are

9

and

27

and their sum is

36

. Using an arithmetic progression instead would give

30

and

57

; and taking only the middle value

27

ignores that two means are asked for.

Original question written in the NDA 2019 Maths pattern and checked twice for the correct answer. Not an official question paper. How we write and check questions · Corrections policy · Report a mistake