$=\frac{1}{2} \mathrm{Y}\left(\frac{\Delta \ell}{\ell} \mathrm{A}\right)(\mathrm{A} \ell)=\frac{1}{2} \mathrm{Y}\left(\frac{\Delta \ell}{\ell} \mathrm{A}\right)\left(\frac{\Delta \ell}{\ell} \times \ell\right)$
$=\frac{1}{2} \mathrm{Y}(\alpha \text { t } \mathrm{A})(\alpha \text { t } \ell)[\therefore \Delta \ell=\ell \alpha \mathrm{f}]$
$=\frac{1}{2}(\mathrm{YA} \propto \mathrm{t})(\ell \propto \mathrm{t})$

