[tex]a^2+b^2-4a+6b+13=0 \\ (a^2-4a+4)+(b^2+6b+9)=0 \\ (a-2)^2+(b+3)^2=0 \\ \\ (a-2)^2 \geq 0~si~(b+3)^2 \geq 0~\Rightarrow (a-2)^2+(b+3)^2 \geq 0,~dar~ \\ (a-2)^2+(b+3)^2=0 \Rightarrow~(a-2)^2=0~si~(b+3)^2=0. \\ \\ (a-2)^2=0 \Rightarrow a=2. \\ (b+3)^2 =0 \Rightarrow b=-3. \\ \\ a+b=2+(-3)=-1.[/tex]