The second and third terms of a geometric progression are 10 and 8 respectively.
Find the sum to infinity.
Solution
Let the first term be \(a\) and the common ratio be \(r\).
The second term is \(ar = 10\) and the third term is \(ar^2 = 8\).
From \(ar^2 = 8\) and \(ar = 10\), we have:
\(\frac{ar^2}{ar} = \frac{8}{10}\)
\(r = 0.8\)
Substitute \(r = 0.8\) into \(ar = 10\):
\(a \times 0.8 = 10\)
\(a = \frac{10}{0.8} = 12.5\)
The sum to infinity \(S_\infty\) is given by:
\(S_\infty = \frac{a}{1-r}\)
\(S_\infty = \frac{12.5}{1-0.8}\)
\(S_\infty = \frac{12.5}{0.2}\)
\(S_\infty = 62.5\)
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