`b)` Ta có :
`9^8 . 5^16 = (3^2)^8 . 5^16 = 3^16 . 5^16 = (3.5)^16 = 15^16`
`=> 15<19 ; 16<20`
`=>15^16 < 19^20`
`=>9^8 . 5^16 < 19^20`
`c)` Ta có :
`3^54 = (3^6)^9 = 729^9`
`2^81 = (2^9)^9 = 512^9`
`=> 729 > 512 ; 9=9`
`=> 729^9 > 512^9`
`=>3^54 > 2^81`