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Problem 235

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An Arithmetic Geometric sequence

Given is the arithmetic-geometric sequence u(k) = (900-3k)r^k-1.
Let s(n) = Σ_k=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.

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等差比数列

已知等差比数列u(k) = (900-3k)r^k-1。
记s(n) = Σ_k=1…nu(k)。

求使得s(5000) = -600,000,000,000的r值。

将你的答案四舍五入至小数点后12位小数。

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