A geometric progression has a third term of 20 and a sum to infinity which is three times the first term. Find the first term.
Solution
Let the first term be \(a\) and the common ratio be \(r\).
The third term is given by \(ar^2 = 20\).
The sum to infinity is \(\frac{a}{1-r} = 3a\).
From the sum to infinity equation, \(\frac{a}{1-r} = 3a\), we get:
\(\frac{1}{1-r} = 3\)
\(1 = 3(1-r)\)
\(1 = 3 - 3r\)
\(3r = 2\)
\(r = \frac{2}{3}\)
Substitute \(r = \frac{2}{3}\) into \(ar^2 = 20\):
\(a \left(\frac{2}{3}\right)^2 = 20\)
\(a \cdot \frac{4}{9} = 20\)
\(a = 20 \times \frac{9}{4}\)
\(a = 45\)
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