`#White`

Bài `1:`

`1)`

`-5x^2y^4(3x^2y^3-2x^3y^2-xy)`

`= -15x^4y^7+10x^5y^6+5x^3y^5`

`2)`

`(x-5)(-x^2+x+1)`

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

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

`= -x^3+6x^2-4x-5`

Bài `2:`

`a)`

`6x(2x-7)-(3x-5)(4x+7)`

`= 12x^2-42x-12x^2-21x+20x+35`

`= (12x^2-12x^2)+(-42x-21x+20x)+35`

`= -43x+35`

Thay `x=-2` vào ta có:

`= -[43. (-2)]+35`

`= 86+35` 

`= 121`

`b)`

`4((3)/(4)x-1)+(12x^2-3x) : (-3x)-(2x-1)`

`= 3x-4+[(12x^2 : (-3x)]+[(-3x) : (-3x)]-2x+1`

`= 3x-4-4x+1-2x+1`

`= (3x-4x-2x)+(-4+1+1)`

`= -3x-2`

Thay `x=3` vào ta có:

`= (-3). 3-2`

`= -9-2`

`= -11`

Đáp án:

Giải thích các bước giải:

 Bài 1:

`a)-5x^2 y^4 (3x^2 y^3 -2x^3 y^2 -xy)`

`=-5x^2 y^4 .3x^2 y^3 -5x^2 y^4.(-2x^3 y^2) -5x^2 y^4.(-xy)`

`=-15x^4 y^7 +10x^5 y^6 +5x^3 y^5`

`b)(x-5)(-x^2 +x+1)`

`=x.(-x^2) +x.x +x.1 -5.(-x^2) -5.x -5.1`

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

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

`=-x^3 +6x^2 -4x-5`

Bài 2:

`a)6x(2x-7)-(3x-5)(4x+7)`

`=6x.2x +6x.(-7) - (3x.4x +3x.7 -5.4x -5.7)`

`=12x^2 -42x - (12x^2 +21x -20x -35)`

`=12x^2 -42x - [12x^2 +(21x -20x )-35]`

`=12x^2 -42x -12x^2 - x +35`

`=(12x^2 -12x^2 ) + (-42x -x) +35`

`=-43x +35`

Thay x=-2

=> `-43.(-2) +35`

`=86 + 35`

`=121`

`b)4(3/4 x -1) + (12x^2 -3x) : (-3x)-(2x-1)`

`=4. 3/4x +4.1 + 12x^2 : (-3x) - 3x : (-3x) - 2x+1`

`=3x +4 -4x +1 -2x+1`

`=(3x-4x-2x) + (-4+1+1)`

`=-3x -2`

Thay x=3

=> `-3.3-2`

`=-9 -2`

`=-11`