Bài `5`:

`a) 78^12 - 78^11 = 78^11 . ( 78 - 1 ) = 78^11 . 77`

`78^11 - 78^10 = 78^10 . ( 78 - 1 ) = 78^10 . 77`

`Do  78^11 > 78^10 => 78^12 - 78^11 > 78^11 - 78^10`

`b) A = 72^45 - 72^44 = 72^44 . ( 72 - 1 ) = 72^44 . 71`

`B = 72^44 - 72^43 = 72^43 . ( 72 - 1 ) = 72^43 . 71`

`Do  72^44 > 72^43 => A > B`

Bài `6:`

`3^39 < 3^42 = ( 3^6 )^7 = 729^7`

`11^21 = ( 11^3 )^7 = 1331^7`

`Do  1331 > 729 => 1331^7 > 729^7`

`=> 11^21 > 3^42 > 3^39`