I have been asked to say where the following derivative is defined;

f'(x)=cosx-sinx for 2kPi-Pi/2<x<2kPi+Pi/2

=cosx+sinx for 2kPi+Pi/2<x<2kPi+3Pi/2

Im pretty sure i need to be using limits as I hav seen from a similar question but I am unsure how?

Thanks for any help