Bài 2:
`a)A=126.(126-152)+5776`
`=126²-126.152+5776`
`=126²-2.126.76+76²`
`=(126-76)²`
`=50²`
`=2` `500`
`b)B=3^16 .7^16-(21^8-2)(21^8+2)`
`=(3.7)^16-[(21^8)^2-2^2]`
`=21^16-(21^16-4)`
`=21^16-21^16+4`
`=4`
Bài 3:
`B=(7+1)(7^2+1)(7^4+1)(7^8+1)`
`=[6(7+1)(7^2+1)(7^4+1)(7^8+1)]/6`
`=[(7-1)(7+1)(7^2+1)(7^4+1)(7^8+1)]/6`
`=[(7^2-1)(7^2+1)(7^4+1)(7^8+1)]/6`
`=[(7^4-1)(7^4+1)(7^8+1)]/6`
`=[(7^8-1)(7^8+1)]/6`
`=(7^16-1)/6`
Ta có:`7^16-1>(7^16-1)/6`
`⇒7^16-1>(7+1)(7^2+1)(7^4+1)(7^8+1)`
`⇒A>B`
Vậy `A>B`