a. 31^5 < 32^5 = ( 2^5)^5=2^25
suy ra 31^5 < 2^25
17^7 > 16^7 = (2^4)^7= 2^28
suy ra 2^28 < 17^7
Vì 31^5 < 2^25 < 2^28 < 17^7
suy ra 31^5 > 17^7
b. có : A = ( 8^9+12/8^9+7) - 1
= 5/8^9 + 7
B = (8^10+4/8^10-1) - 1
= 5/8^10-1
8^9 + 7 < 8^10 - 1 nên 5/8^9+7 > 5/8^10-1