Question

the interval at which cosine graph repeat itself is

Answer 1

2π. If you are familiar with the unit circle, you could apply this knowledge. A full cycle of the unit circle is 2π, and then it repeats. Very useful information to memorize if you will be using trig!

Try reading this and making sense of it if you want trig to become loads easier!
http://en.wikipedia.org/wiki/Unit_circle