picanha=Plot3D[{1+(x^2),3-(x^2)},{x,-1,1},{y,-1,1},Mesh->False,Boxed->False,AspectRatio->Automatic,PlotStyle->Opacity[0.3]]

picanha2=RegionPlot3D[1+(x^2)≤ z≤ 3-(x^2),{x,-1,1},{y,-1,1},{z,1,3},Mesh->False,Boxed->False,PlotStyle->Opacity[0.3],PlotPoints->30]

fatia=ParametricPlot3D[{x,0,(1-t)(1+(x^2))+t(3-(x^2))},{x,-1,1},{t,0,1},PlotStyle->{LightGreen,Opacity[0.3]}]

Show[picanha,fatia]

Show[picanha2,fatia]