
$\mathrm{V}_{\mathrm{xy}}=(2 \mathrm{I})(\mathrm{R})$
so energy stored $=\frac{1}{2} \mathrm{C}\left(\mathrm{V}_{\mathrm{xy}}\right)^{2}$
$\boxed{{\text{U}} = \frac{1}{2}{\text{C}} \times 4{{\text{I}}^2}{{\text{R}}^2} = 2{\text{C}}{{\text{I}}^2}{{\text{R}}^2}}$
