^=la puterea
[4^n*5^(n+1) +4^(n+1)*5^n] / [3^n*7^(n+1)+3^(n+1)*7^n-21^n] =
=[4^n*5^n(5+4] / [3^n*7^n(7+3-1)] =
=4^n*5^n * 9 / 3^n*7^n *9 = deci se simplifica cu 9
=4^n*5^n / 3^n*7^n
[6^(n+1)+6^n] /[5^(n+2)-6*5^n]=
=[6^n(6+1)] /[5^n(5²-6]=
=6^n*7 /5^n *19 deci ireductibila
