[tex]\it \dfrac{\overline{ab5}+12}{\overline{2ab}+123}=1 \Rightarrow \overline{ab5}+12=\overline{2ab}+123 \Rightarrow 10\overline{ab}+5+12=200+\overline{ab}+123 \Rightarrow \\ \\ \\ \Rightarrow 9\overline{ab}=306|_{:9} \Rightarrow \overline{ab}=34 \Rightarrow a+b=3+4=7[/tex]
