7.
[tex]\it n=\overline{a7ab}\\ \\ a+b\leq3\ \Rightarrow (a,\ b)\in\{(1,\ 0);\ (1,\ 1);\ (1,\ 2);\ (2,\ 0);\ (2,\ 1)\}\\ \\ \\ n\in\{1710,\ \ 1711,\ \ 1712,\ \ 2720,\ \ 2721 \}[/tex]
6.
[tex]\it =5^{14}(6+9-10)=5^{14}\cdot5=5^{15}=(5^5)^3\ \longrightarrow\ cub\ \ perfect[/tex]
