The <a href="https://en.wikipedia.org/wiki/Radians" class="extiw" title="w:Radians">radian</a> and its relation to the <a href="https://en.wikipedia.org/wiki/circle" class="extiw" title="w:circle">circle</a>.
<a href="//commons.wikimedia.org/wiki/User:LucasVB" title="User:LucasVB">Lucas Vieira</a>
2013-03-14
<span class="int-own-work" lang="en">Own work</span>