[tex]\it \mathcal{A}_{\triangle} = \dfrac{\ell^2\sqrt3}{4} =72\sqrt3 |_{:\sqrt3} \Longrightarrow \dfrac{\ell^2}{4} =72 \Longrightarrow \ell^2= 4\cdot72 =4\cdot36\cdot2 [/tex]
[tex]\it \Longrightarrow \ell = 12\sqrt2 cm[/tex]
[tex]\it R = \dfrac{\ell\sqrt3}{3}[/tex]
[tex]\it \ell_6 = R,\ \ a_6 = \dfrac{R\sqrt3}{2}[/tex]
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