Plot[{Sin[9*x]*Sin[x],Sin[x],-Sin[x]},{x,0,4*Pi},PlotStyle->{Normal,Dashed,Dashed},PlotLabel->None,AxesLabel->None]

Plot[{x*Sin[9x],x,-x},{x,0,3*Pi}, PlotStyle->{Normal, Dashed, Dashed}, PlotLabel->None, AxesLabel->None]

Series[ArcTan[x],{x,t,6}]

Plot[21*x^6-105*x^4-63*x^2-6,{x,0,1}]