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1.6.15 Standard formulae for geometric series (A-level only)

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Question 14

The second term of a geometric series is 3.75 and the sum to infinity is 20.

a.

Find the two possible values of rrr.

[4]
b.

Find the corresponding two possible values of aaa.

[2]
c.

Given that r r\,r takes the larger of the two possible values, find the smallest value of n n\,n for which Sn>19S_n > 19Sn​>19.

[4]

1.6.15 Standard formulae for geometric series (A-level only) Questions

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