Question 1060248
<pre>
The reference angle is the acute angle whose cosine is 1/2.

We can find it on the unit circle:

 *[illustration unitcircle.jpg].

It is {{{pi/3}}}

The cosine is negative in quadrants II and III, so we find
those two angles on the unit circle as 

{{{2pi/3}}} and {{{4pi/3}}}

Edwin</pre>