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June 2016 p11 q9
908
The first term of a geometric progression in which all the terms are positive is 50. The third term is 32. Find the sum to infinity of the progression.
Solution
Let the first term be denoted by \(a = 50\) and the common ratio by \(r\).
The third term of the geometric progression is given by \(ar^2 = 32\).
Substituting the value of \(a\), we have:
\(50r^2 = 32\)
Solving for \(r^2\), we get:
\(r^2 = \frac{32}{50} = \frac{16}{25}\)
Thus, \(r = \frac{4}{5}\) (since all terms are positive).
The sum to infinity of a geometric progression is given by:
\(S_\infty = \frac{a}{1 - r}\)
Substituting the values of \(a\) and \(r\), we have: