Given the task definition and input, reply with output. In this task, you are given a question in Persian, and your task is to translate it into English.

مقدار این چیست؟ [math]\displaystyle\int_{0}^{\infty} \dfrac{x}{x^2+a^2}\,dx [/math]

What is the value of [math]\displaystyle\int_{0}^{\infty} \dfrac{x}{x^2+a^2}\,dx [/math]?