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June 2015 p11 q7
912
The third and fourth terms of a geometric progression are \(\frac{1}{3}\) and \(\frac{2}{9}\) respectively. Find the sum to infinity of the progression.
Solution
Let the first term be \(a\) and the common ratio be \(r\).
The third term is \(ar^2 = \frac{1}{3}\) and the fourth term is \(ar^3 = \frac{2}{9}\).
Divide the fourth term by the third term to find \(r\):
\(\frac{ar^3}{ar^2} = \frac{2/9}{1/3}\)
\(r = \frac{2}{3}\)
Substitute \(r = \frac{2}{3}\) into \(ar^2 = \frac{1}{3}\):