I want to plot $f(x, y) = x^2 + y^2$? I can plot functions of a single variable but I don't know how to plot multivariable function.
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$\begingroup$Your plot has three dimensions. You can plot it in perspective, in contour lines, or in various other representations. Alpha gives the following:
The graph of this function will be a surface in space. Above the point $(x,y)$ in the plane it has height $f(x,y)$.
$\endgroup$ $\begingroup$In order to sketch this surface we need to consider$$z=x^2+y^2=(\text{distance of }(x,y)\text{ from }(0,0))^2$$So this is intuitively equivalent to the surface created by rotating the graph of $z=x^2$ about the $z$ axis.
$\endgroup$ $\begingroup$Since the function $f(x,y)=x^2+y^2$ is from $\mathbb{R^2}$ to $\mathbb{R}$ we clearly need to visualise it in $\mathbb{R^{2+1}}$,and since the domain is $\mathbb{R^{2}}$ the graph will be a surface!! If you want to visualise any such fuction you can use and your graph looks like
PS:I would suggest you to use LaTeX to type your questions.
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