Mathematics

Mathematics

Problem

Given a geometric progression with first term

a

and common ratio

r

, the sum of its first two terms is

30

and the sum of its first four terms is

120

. After eliminating

a

, which one of the following equations in

r

must be satisfied? a)

1+r^{2}=4

b)

1+r+r^{2}+r^{3}=4(1+r)

c)

1+r+r^{2}=4(1+r)

d)

1+r+r^{2}+r^{3}=4r

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