Ok so there's this maths problem I can't solve:

What I don't get it,why is it from -pi/4 to pi/4,the graph takes that shape?Should it be that quardrant part of the cos2x graph?Since the want it until pi/4 only?

The same question also asked me to find the fourier series of the graph up to the 4th harmonic,but I'm not sure how to do that to,since I don't have the 0<t<pi/4 range.

Thanks for your help!