Needs["Calculus`FourierTransform`"];
f=If[x<0,30+100x+100x^2,30-50x];
reihe2=NFourierTrigSeries[f,x,2,AccuracyGoal->4];
reihe10=NFourierTrigSeries[f,x,10,AccuracyGoal->4];
bild=Plot[{f,reihe2,reihe10},{x,-0.5,0.5},
PlotStyle->{{},Dashing[{0.01}],{}},AxesLabel->{"x","y"},
ImageSize->{400,300}];