Integration for finding the Arc Length of Circle $x^2+y^2=a^2$ Question:

Find the arc length of the circle given by $x^2+y^2=a^2$.   
$Ans = 2\pi a$

How to obtain the ans? I have no ideas after doing the following thing.

Thank you for your attention.

Following Mark's comment about converting to polar coordinates, since $r^2=x^2+y^2$, then it immediately follows that $r=a$, where $a$ is  a constant.  The arc length formula for polar coordinates is given by 
$$L=\int\sqrt{r^2+\left(\dfrac{dr}{d\theta}\right)^2}\mathrm d\theta$$
Since $r=a$, then $\dfrac{dr}{d\theta}=0$ because the derivative of a constant is always $0$. Then, the formula becomes (for arc length of a circle, which is $0\le\theta\le 2\pi$)
$$L=\int_0^{2\pi}\sqrt{r^2}\mathrm d\theta=\int_0^{2\pi}r\mathrm d\theta=\left.r\theta\right|_{\theta=0}^{\theta=2\pi}=2\pi r-0=2\pi a.$$
