ContourPlot3D[{x^2+y^2==1,1/3*x^2+4/3*y^2+z^2=1},{x,-2,2},{y,-2,2},{z,-2,2}]

Uh oh! Wolfram|Alpha doesn't run without JavaScript.

Please enable JavaScript. If you don't know how, you can find instructionshere.Once you've done that, refresh this page to start using Wolfram|Alpha.