Line integral polar coordinates

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What I am uncertain of is how this was derived? I know that $\theta=\arctan(y/x)$ but i dont get how we get the dy/dx inside the arctan. $$\oint\limits_{c}\,\mathrm{d}\theta=\oint\limits_{c} \mathrm{d}\arctan(\mathrm{dy}/\mathrm{dx}) $$