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Bài `4:`

`a,A=(5x-1)(x+3)-(x-2)(5x-4)`

`=5x^2+15x-x-3-5x^2+4x+10x-8`

`=(5x^2 -5x^2)+(15x -x+4x+10x)+(-3-8)`

`=28x -11`

`B=(3a-2b)(9a^2 +6ab+4b^2)`

`=(3a)^3 -(2b)^3`

`=27a^3 -8b^3`

`b,`

`n(2n-3)-2n(n+2)`

`=n . 2n-n . 3 -2n . n -2n  .2`

`=2n^2-3n-2n^2-4n`

`=(2n^2-2n^2)+(-3n-4n)`

`=0-7n`

`=-7n`

Vì `-7 \vdots 7` nên `-7n \vdots 7`

Vậy `n(2n-3)-2n(n+2)` luôn chia hết cho `7` với mọi số nguyên `n`