# Problem 235: An Arithmetic Geometric sequence

![graphic](img235.gif)

Given is the arithmetic-geometric sequence u(k) = (900-3k)rk-1. Let s(n)
= Σk=1...nu(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.