[tex]\mathcal{A}_l=\dfrac{\mathcal{A}_t}{2} \Longrightarrow \mathcal{A}_t =2 \mathcal{A}_l[/tex]
[tex]\mathcal{A}_t =\mathcal{A}_l + 2\mathcal{A}_b \ \ \ \ (2)
\\\;\\
(1), (2) \Longrightarrow 2\mathcal{A}_l =\mathcal{A}_l + 2\mathcal{A}_b\Longrightarrow \mathcal{A}_l = 2\mathcal{A}_b [/tex]
[tex]2\pi RG = 2\pi R^2 \Longrightarrow G = R.[/tex]