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Let $x_1$ and $x_2$ be two real numbers and define the column vector $\mathbf{x}=[x_1,x_2]$. Let $\mathbf{A}_1$ and $\mathbf{A}_2$ be two $2\times 2$ real symmetric matrices. Then I need to plot the surfaces \begin{align} \mathbf{x}^T\mathbf{A}_1\mathbf{x}+1 &\leq 0 \\ \mathbf{x}^T\mathbf{A}_2\mathbf{x}+1 &\leq 0 \end{align}

How do I do this in Mathematica?

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  • $\begingroup$ Please try to write some code. $\endgroup$ – Dr. belisarius Aug 11 '14 at 4:13
  • $\begingroup$ @belisarius I really want to and I understand it is inappropriate to ask without trying. I come from a matlab & engineering background and doing this in matlab is a pain. Just started with mathematica 2 hrs back. $\endgroup$ – dineshdileep Aug 11 '14 at 5:32
  • $\begingroup$ Well, you've a kickstart below $\endgroup$ – Dr. belisarius Aug 11 '14 at 16:51
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SeedRandom[84];
a = # + Transpose@# &@RandomReal[{0, 1}, {2, 2}];
RegionPlot[{x, y}.a.{x, y} > 0, {x, -2, 2}, {y, -2, 2}]

Mathematica graphics

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  • $\begingroup$ +1, but the condition is $x^TAx+1\le0$, not $x^TAx>0$. $\endgroup$ – Rahul Aug 11 '14 at 6:33
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    $\begingroup$ @RahulNarain Nobody is perfect :) $\endgroup$ – Dr. belisarius Aug 11 '14 at 7:07
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Using V10 functionality and borrowing from Belisarius:

SeedRandom[84]
a = # + Transpose@# &@RandomReal[{0, 1}, {2, 2}];

We create an ImplicitRegion

region = ImplicitRegion[First[{x, y}.a.{{x}, {y}}] > 0, {x, y}];

And discretize it:

DiscretizeRegion[region, {{-2, 2}, {-2, 2}}]

Mathematica graphics

If you want Frame:

Show[%, PlotRange -> {{-2, 2}, {-2, 2}}, Frame -> True]

Mathematica graphics

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