<a href="https://en.wikipedia.org/wiki/Langley%E2%80%99s_Adventitious_Angles" class="extiw" title="en:Langley’s Adventitious Angles">en:Langley’s Adventitious Angles</a>: Prove that angle BEF = 30°
<a href="//commons.wikimedia.org/wiki/User:David_Eppstein" class="mw-redirect" title="User:David Eppstein">David Eppstein</a>
<span class="int-own-work" lang="en">Own work</span>