[tex] \frac{4^{n}5^{n+1}+4^{n+1}5^{n} }{3^{n}7^{n+1}+3^{n+1}7^{n}-21^{n}} = \\ \\\frac{4^{n}5^{n}5+4^{1} 4^{n}5^{n} }{3^{n}7^{n}7+3^{n}3^{1} 7^{n}-21^{n}}= \\ \\\frac{ 20^{n}(5+4)}{21^{n}(7+3-1) } = \\ \\ = \frac{20^{n}9}{21^{n}9 } [/tex]
evident se simplifica prin 9
