Help with trig problem: exact value of cos^-1(cos(4pi/3))

[Guest]

I need some help getting through the following trig problem:

Find the exact value of cos^-1(cos(4pi/3))

Thanks

 

[pka]

If I have read the problem correctly then:
\(\displaystyle \L


\arccos \left( {\cos \left( {\frac{{4\pi }}{3}} \right)} \right) = \arccos \left( { - \frac{1}{2}} \right) = \frac{{2\pi }}{3}\)