a) Ta có
$2^{225} = 2^{3.75} = (2^3)^{75} = 8^{75}$
$3^{150} = 3^{2.75} = (3^2)^{75} = 9^{75}$
Ta có $8 < 9$ nên $8^{75} < 9^{75}$
Vậy $2^{225} < 3^{150}$.
b) Ta có
$2^{91} = 2^{13.7} = (2^{13})^7$
$5^{35} = 5^{5.7} = (5^5)^7$
Ta có
$2^{13} = 8192>3125=5^5$
Vậy $(2^{13})^7 > (5^5)^7$ hay $2^{91} > 5^{35}$.
c) Ta có
$99^{20} = 99^{2.10} = (99^2)^{10} = 9801^{10} < 9999^{10}$
Vậy $99^{20} < 9999^{10}$.