Also if you do not know the value of the sine and cosine at interger multiples of pi you are not ready to study Fourier series and need to repeat trig.
It is a little weird, imho, to make this kind of trivial questions in very basic trigonometry
if you're dealing with Fourier series...even if you don't remember. and this is already weird,
why don't you check your high school books or, at least, google it?
The sine of any multiple of pi is zero...