Two rods one of aluminum length I₁, having coefficient of linear expansion ${\mathrm{\alpha }}_1$ and other of steel of length l₂ having coefficient of linear expansion ${\mathrm{\alpha }}_2$ are joined end to the end. The expansion in both the rods is same for the same variation in temperature. Then the value ${\mathrm{l}}_1/\left({\mathrm{l}}_1+{\mathrm{l}}_2\right)$ is