(II) (a) Show that the change in the density $\rho$ of a substance, when the temperature changes by $\Delta T,$ is given by $\Delta \rho=-\beta \rho \Delta T .$ (b) What is the fractional change in density of a lead sphere whose temperature decreases from $25^{\circ} \mathrm{C}$ to $-55^{\circ} \mathrm{C} ?$