Oh, I see what you were asking. Sorry.:smile:
##a_n = \frac{2}{2}\int_{-1}^{1}f(t)cos(n\omega t) \, dt= \int_{-1}^{0}(1+t)cos(n\omega t) \, dt + \int_{0}^{1}(1-t)cos(n\omega t) \, dt ,##
##=\int_{-1}^{0}cos(n\omega t) \, dt +\int_{-1}^{0}tcos(n\omega t) \, dt +\int_{0}^{1}cos(n\omega t) \, dt...