[tex]a^2+b^2-4a+10b+20 = 0 \\ a^2-4a+4+b^2+10b+25 - 9 = 0\\ (a-2)^2+(b+5)^2 =9\Leftrightarrow\\ \Leftrightarrow \Big\{a-2=0 $ si $ b+5 = 3\Big\} $ sau $ \Big\{a-2 = 3 $ si $ b+5 = 0\Big\}\\ \\\text{sau} $ $\Big\{a-2 = 0 $ si $ b+5 = -3\Big\} $ sau $ \Big\{a-2 = -3 $ si $ b+5 = 0\Big\} \\ \\ \Leftrightarrow \Big\{a=2 $ si $ b=-2\Big\} $ sau $ \Big\{a=5$ si $ b=-5\Big\}\\ \\\text{sau} $ $\Big\{a=2 $ si $ b=-8\Big\} $ sau $ \Big\{a=-1 $ si $ b=-5\Big\} \\ \\[/tex]
[tex]\Rightarrow (a,b) = \Big\{(2,-2);(5,-5);(2,-8);(-1,-5)\Big\} \\ \\ 2\ \textgreater \ -2, 5\ \textgreater \ -5,2\ \textgreater \ -8,-1\ \textgreater \ -5\\ \\ \Rightarrow a\ \textgreater \ b[/tex]
