The variation of length of a substance along three principle directions with temperature is shown in the figure. The average superficial expansivity of the substance is

$\begin{array}{l}{\alpha }_{\mathrm{x}}=\frac{2004.0-2000.0}{2000.0\times (10-0)}=2\times {10}^{-4}{/}^{\circ }\mathrm{C}\\ {\alpha }_{\mathrm{y}}=\frac{2003.2-2000.0}{2000.0\times (10-0)}=1.6\times {10}^{-4}{/}^{\circ }\mathrm{C}\\ {\alpha }_{\mathrm{z}}=\frac{2000.8-2000.0}{2000.0\times 10}=0.4\times {10}^{-4}{/}^{\circ }\mathrm{C}\\ \alpha =\frac{{\alpha }_{\mathrm{x}}+{\alpha }_{\mathrm{y}}+{\alpha }_{\mathrm{z}}}{3}=\frac{(2+1.6+0.4)\times {10}^{-4}}{3}==\frac{4}{3}\times {10}^{-4}{/}^{\circ }\mathrm{C}\\ \beta =2\alpha =26.67\times {10}^{-5}{/}^{\circ }\mathrm{C}\end{array}$