Q53 of 74 Page 9

Find four numbers forming a Geometric progression in which the third term is greater than the first by 9, and the second term is greater than the fourth by 18.


The four terms of G.P are a, ar, ar2, ar3.

ar2 - a = 9

a (r2-1) = 9 ………..(1)

ar (1 - r2) = 18 ……..(2)

Dividing (2) by (1)





r = -2

Substitute r in (1)

a (4-1) = 9

a = 3

... the four numbers are 3, 3(-2), 3(-2)2 and 3(-2)3

(i.e) 3, -6, 12, -24.

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