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Đáp án+Giải thích các bước giải:

 `a)`

`(x+2)^{3}-(x+1)^{3}`

`=[(x+2)-(x+1)].[(x+2)^{2}+(x+2).(x+1)+(x+1)^{2}]`

`=(x+2-x-1).(x^{2}+4x+4+x^{2}+x+2x+2+x^{2}+2x+1)`

`=1.(3x^{2}+9x+7)`

`=3x^{2}+9x+7`

`b)`

`(x-3)^{3}-x.(x-3)^{2}`

`=(x-3)^{2}.(x-3-x)`

`=(x^{2}-2.x.3+3^{2}).(-3)`

`=(x^{2}-6x+9).(-3)`

`=-3x^{2}+18x-27`

`c)`

`(x+1)^{3}-x^{2}.(x+1)-2x.(x+2)`

`=(x+1).[(x+1)^{2}-x^{2}]-2x^{2}-4x`

`=(x+1).(x+1-x).(x+1+x)-2x^{2}-4x`

`=(x+1).1.(2x+1)-2x^{2}-4x`

`=(x+1).(2x+1)-2x^{2}-4x`

`=2x^{2}+x+2x+1-2x^{2}-4x`

`=-x+1`