Problem 1165 (difficulty: 6/10)

\(\displaystyle \frac{x}1 - \frac{x^3}3 + \frac{x^5}5 - \frac{x^7}7 +- \ldots = ? \qquad \sum_{k=0}^\infty\frac{(-1)^k}{4k+1} = ? \)

Give me another random problem!

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Request for a concrete problem:I want problem no.

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