Describe the surfaces defined by the equation f = const, where f = x^2 + 4y^2.
Hint: A good way to do this is to plot the surfaces parametrically using a program like Mathematica (in Mathematica, use ParametricPlot3D). In order to parametrize this function, picture it this way: x^2 + 4y^2 = constant = C^2 (saying that this constant is C2 is a matter of convenience, you'll see why in the next step)
x^2/C^2 + 2^2y^2/C^2 = 1
now we define 2 new coordinates: u = x/C, v = 2y/C. So we have:
u^2+v^2=1
Now this curve is very similar to one we have already seen, that we can parametrize as a function of a parameter θ. After parametrizing u=u(θ) and v=v(θ), find x=x(θ) and y=y(θ), and you are ready to plot it.