bild1=Plot[Sqrt[mu],{mu,0,2},AxesLabel->{"mu","x"},ImageSize->{400,300}];
bild2=Plot[-Sqrt[mu],{mu,0,2},PlotStyle->{{Dashing[{0.01}]}},ImageSize->{400,300}];
abb=Show[bild1,bild2,ImageSize->{400,300}];