$\begin{align}  & \Delta \theta =40 \\  & {{L}_{1}}=1200cm \\  & \beta =3\times {{10}^{-5}}\to \alpha ={{10}^{-5}} \\  & \Delta L={{L}_{1}}\times \alpha \times \Delta \theta =1200\times {{10}^{-5}}\times 40=0/48 \\ \end{align}$