i think you mean when do we use degree and when do we use radian?
generally, for most questions that we do now, it is in radian
for example in the topic of calculus
sorry, your question needs to be more specific for us to help
Check out the library/school library
Look for HSC Study Pack "Quanta to Quarks" An HSC Option Book
by John Blackmore and Elizabeth Blackmore
It is pink in colour
It is quite good and it got all you need in it
In fact, more than what you need
And also check out the HSC online website