Project Euler/235
From charlesreid1
Problem Statement
An Arithmetic Geometric Sequence
Given is the arithmetic-geometric sequence u(k) = (900-3k) r^{k-1}.
Let s(n) = Σ_{k=1...n} u(k).
Find the value of r for which s(5000) = -600,000,000,000.
Give your answer rounded to 12 places behind the decimal point.
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