# Primitive of Arctangent of x over a over x

## Theorem

 $\displaystyle \int \frac 1 x \arctan \paren {\frac x a} \rd x$ $=$ $\displaystyle \sum_{k \mathop = 0}^\infty \paren {-1}^k \frac {x^{2 k + 1} } {\paren {2 k + 1}^2 a^{2 k + 1} }$ $\displaystyle$ $=$ $\displaystyle \frac x a - \frac {x^3} {3^2 a^3} + \frac {x^5} {5^2 a^5} - \frac {x^7} {7^2 a^7} + \cdots + C$

## Proof

 $\displaystyle \int \frac 1 x \arctan \paren {\frac x a} \rd x$ $=$ $\displaystyle \int \frac 1 x \sum_{k \mathop = 0}^\infty \paren {-1}^k \frac 1 {2 k + 1} \paren {\frac x a}^{2 k + 1} \rd x$ Power Series Expansion for Real Arctangent Function $\displaystyle$ $=$ $\displaystyle \sum_{k \mathop = 0}^\infty \paren {-1}^k \frac 1 {\paren {2 k + 1} a^{2 k + 1} } \int x^{2 k} \rd x$ Fubini's Theorem $\displaystyle$ $=$ $\displaystyle \sum_{k \mathop = 0}^\infty \paren {-1}^k \frac{x^{2 k + 1} }{\paren {2 k + 1}^2 a^{2 k + 1} } + C$

$\blacksquare$