`1)3x(x²-5x+7)-(x²-5x)`

 `=3x³-15x²+21x-x²+5x`

`=3x³-(15x²+x²)+(21x+5x)`

`=3x³-16x²+26x`

`2)(-x+4)(x-3)-7x(x²+2x+2)`

`=-x²+3x+4x-12-7x³-14x²-14x`

`=-7x³+(-x²-14x²)+(3x+4x-14x)-12`

`=-7x³-15x²-7x-12`

`3)2x(x-1)(-2x+3)-(x-7)(-2x+1)`

`=(2x²-2x)(-2x+3)-(-2x²+x+14x-7)`

`=-4x³+6x²+4x²-6x+2x²-x-14x+7`

`=-4x³+(6x²+4x²+2x²)-(6x+x+14x)+7`

`=-4x³+12x²-21x+7`

`4)(8x-1)(2x-3)+(x-2)(7-x)`

`=16x²-24x-2x+3+7x-x²-14+2x`

`=(16x²-x²)-(24x+2x-7x-2x)+(3-14)`

`=15x²-17x-11`

`5)-(x+3)(2x-5)+(x-6)(3x-1)`

`=-(2x²-5x+6x-15)+3x²-x-18x+6`

`=-2x²+5x-6x+15+3x²-x-18x+6`

`=(-2x²+3x²)+(5x-6x-x-18x)+(15+6)`

`=x²-20x+21`

`6)-2x(2x²+x+3)+(4x-2)(-3x+1)`

`=-4x³-2x²-6x-12x²+4x+6x-2`

`=-4x³-(2x²+12x²)+(-6x+4x+6x)-2`

`=-4x³-14x²+4x-2`

`7)x³(x-7)-(x²+2)(3-2x²)`

`=x^4-7x³-(3x²-2x^4+6-4x²)`

`=x^4-7x³-3x²+2x^4-6+4x²`

`=(x^4+2x^4)-7x³+(-3x²+4x²)-6`

`=3x^4-7x³+x²-6`

`x^3 . (x-7) - (x^2 + 2).(3-2x^2)`

` = x^4 - 7x^3 -3x^2 + 2x^4 - 6 + 4x^2`

` = (x^4 + 2x^4) - 7x^3 + (4x^2 - 3x^2)-6`

` = 3x^4 - 7x^3 + x^2 - 6`

Ví dụ `10`

`1) 3x.(x^2 - 5x+7) - (x^2  -5x)`

` = 3x^3 - 15x^2 + 21x - x^2 + 5x`

` = 3x^3 - (15x^2 + x^2) + (21x + 5x)`

` = 3x^3 - 16x^2 + 26x`

`2) (-x+4)(x-3) - 7x(x^2  +2x+2)`

` = -x^2 + 3x + 4x - 12 - 7x^3 - 14x^2 - 14x`

` = -7x^3 -(x^2 + 14x^2) + (3x + 4x - 14x) - 12`

` = -7x^3 - 15x^2 - 7x  -12`

`3) 2x.(x-1)(-2x+3) - (x-7)(-2x+1)`

` = (2x^2 - 2x).(-2x+3) - (-2x^2 + x + 14x - 7)`

` = -4x^3 + 6x^2 + 4x^2 - 6x + 2x^2 - x - 14x + 7`

` = -4x^3 + (6x^2 + 4x^2 + 2x^2) - (6x + 14x+x)  +7`

` = -4x^3 + 12x^2 - 21x + 7`

`4) (8x-1)(2x-3)+(x-2)(7-x)`

` = 16x^2 - 24x - 2x + 3 + 7x - x^2 - 14 + 2x`

` = (16x^2 - x^2) + (2x - 2x - 24x + 7x) + (3-14)`

` = 15x^2 -17x - 11`

`5) -(x+3)(2x-5) + (x-6)(3x-1)`

` = -2x^2 + 5x - 6x + 15 + 3x^2 - x - 18x + 6`

` = (-2x^2 + 3x^2) + (5x-6x - x -18x) + (15+6)`

` = x^2 -20x + 21`

`6) -2x.( 2x^2 + x+3) + (4x-2) . (-3x+1)`

`=  -4x^3 - 2x^2 - 6x - 12x^2 + 4x +6x - 2`

` = -4x^3 - (2x^2 + 12x^2) + (6x-6x + 4x) - 2`

` = -4x^3 - 14x^2 + 4x  -2`