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in a geometric series, the sum of term a2 and a3 is 12

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and that the sum of term a3 and a4 is 60... 1st term is what?

asked Nov 3, 2014 in ALGEBRA 2 by anonymous

1 Answer

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Formula for the nth term in the geometric series an = a rn-1

a2 = ar2 - 1

a2 = ar

a3 = ar3 - 1

a3 = ar2

a4 = a r4 - 1

a4 = ar3

ar + ar2 = 12 ---> (1)

ar2 + ar3 = 60 ---> (2)

Solve the equations (1) and (2).

From (2), r(ar + ar2 ) = 60

From (1), r(12) = 60

r = 60/12

r = 5

Substitute r value in equation (1).

5a + 25a = 12

30a = 12

a = 12/30

a = 2/5.

First term in the sequence a = 2/5.

answered Nov 3, 2014 by david Expert

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