Coefficients of linear expansion of an isotropic solid along three rectangular axes in the solid are ${\mathrm{\alpha }}_{\mathrm{x},}{\mathrm{\alpha }}_{\mathrm{y}} \mathrm{and} {\mathrm{\alpha }}_{\mathrm{z}.}$ Coefficient of cubical expansion of the solid for small change in temperature can be expressed as : ( assume ${\mathrm{\alpha }}_{\mathrm{x}},{\mathrm{\alpha }}_{\mathrm{y}} \mathrm{and} {\mathrm{\alpha }}_{\mathrm{z}}$to be small quantities)