Why Plot[] doesn't work for 9x² + 18xy + 13y² - 4 in Mathematica 7

[cuongktv]

Hey every pro.
Please help me to check with Plot function in Mathematica 7 like this: 9x²+18xy+13y²-4
I only could Plot with "Plot3D[]", but could not get result with "Plot[]" only.
Thanks!

 

[MATLABdude]

Welcome to PhysicsForums!

The plot[] function only works with functions of one variable. There used to be an ImplicitPlot[] function that could handle two, but this seems to now be encapsulated in the CountourPlot[] function:
http://reference.wolfram.com/mathematica/Compatibility/tutorial/Graphics/ImplicitPlot.html
http://reference.wolfram.com/mathematica/ref/ContourPlot.html

EDIT: Oops! ImplicitPlot[] wouldn't have done anything for you either, since you don't have an implicit equation!

 

[cuongktv]

Ok, I got it. Thank you so much!