Calculus inverse trig question

Question

arccos(cos 7π/5)=?

 

the answer is 3π/5 but I do not understand how to get it

 

thankyou for the help!

Answer 1

For arccos to be be a function the domain must be restricted (this should have been learned).

The domain for arccos is 0 to π.

cos 7π/5 is negative (7π/5 is in the QIII).

The arccos must be in QII, that is the 3π5.