cvals=Table[10n+5, {n, 1, 20}];

ContourPlot[{x^2+y^2==cvals,x^2+2y^2==44},{x, -7, 7}, {y, -7, 7},ContourStyle->{Directive[RGBColor[0.,.5,0.7],Opacity[1.],AbsoluteThickness[1.]],Directive[RGBColor[1,0,0],Opacity[1.],AbsoluteThickness[2.]]}]

Plot3D[(x^(2/3))(y^(1/3)), {x, 0, 10}, {y, 0, 10}]

ContourPlot[{(x^(2/3)) (y^(1/3))==Table[n/2,{n,1,10}],x+y==3.78`},{x,0,5},{y,0,5},ContourStyle->{Directive[RGBColor[0,0.5,0.7],Opacity[1.`],AbsoluteThickness[1.]],Directive[RGBColor[1.,0,0],Opacity[1.`],AbsoluteThickness[2.`]]}]