Chọn B

`C4:`

`x/[x-2]+2/[2-x]=x/[x-2]-2/[x-2]=[x-2]/[x-2]=1`

`C5:[15x^2]/[17y^4] . [34y^5]/[15x^3]`

 `=[15x^2 . 2.17y^4 . y]/[17y^4 . 15x^2 . x]=[2y]/x`

`C5:[6x+18]/[(x+4)^2]:[3(x+3)]/[x+4]`

 `=[6(x+3)]/[(x+4)^2] . [x+4]/[3(x+3)]`

`=2/[x+4]`

bạn viết thế này khó nhìn quá

nhìn hơi đau mắt nhá bạn hoa mắt quá

1:

a: \(\left(2x-5\right)^2-4x\left(x+3\right)\)

\(=4x^2-20x+25-4x^2-12x\)

=-32x+25

b: \(\left(x-2\right)^3-6\left(x+4\right)\left(x-4\right)-\left(x-2\right)\left(x^2+2x+4\right)\)

\(=x^3-6x^2+12x-8-\left(x^3-8\right)-6\left(x^2-16\right)\)

\(=-6x^2+12x-6x^2+96=-12x^2+12x+96\)

c: \(\left(x-1\right)^2-2\left(x-1\right)\left(x+2\right)+\left(x+2\right)^2+5\left(2x-3\right)\)

\(=\left(x-1-x-2\right)^2+5\left(2x-3\right)\)

\(=\left(-3\right)^2+5\left(2x-3\right)\)

\(=9+10x-15=10x-6\)

2: 

a: \(\left(2-3x\right)^2-5x\left(x-4\right)+4\left(x-1\right)\)

\(=9x^2-12x+4-5x^2+20x+4x-4\)

\(=4x^2+12x\)

b: \(\left(3-x\right)\left(x^2+3x+9\right)+\left(x-3\right)^3\)

\(=27-x^3+x^3-9x^2+27x-27\)

\(=-9x^2+27x\)

c: \(\left(x-4\right)^2\left(x+4\right)-\left(x-4\right)\left(x+4\right)^2+3\left(x^2-16\right)\)

\(=\left(x-4\right)\left(x+4\right)\left(x-4-x-4\right)+3\left(x^2-16\right)\)

\(=\left(x^2-16\right)\left(-8\right)+3\left(x^2-16\right)\)

\(=-5\left(x^2-16\right)=-5x^2+80\)

\(a,x^2+4x-21-x^2-4x+5=-16\\ b,=\left(x+8-x+2\right)^2=10^2=100\\ c,=x^2\left(x^2-16\right)-\left(x^4-1\right)\\ =x^4-16x^2-x^4+1=1-16x^2\\ d,=x^3+1-x^3+1=2\)

\(a,=x^2+4x-21-x^2-4x+5=-16\\ b,=\left(x+8-x+2\right)^2=10^2=100\\ c,=x^2\left(x^2-16\right)-\left(x^4-1\right)\\ =x^4-16x^2-x^4+1=1-16x^2\\ d,=x^3+1-x^3+1=2\)

a) 2x( x - 7 ) - ( x + 3 )( x - 2 ) - ( x + 4 )( x - 4 )

= 2x² - 14x - ( x² + x - 6 ) - ( x² - 16 )

= 2x² - 14x - x² - x + 6 - x² + 16

= 22 - 15x

b) ( 2x + 5 )( x - 2 ) - 3( x + 2 )² + ( x + 1 )²

= 2x² + x - 10 - 3( x² + 4x + 4 ) + x² + 2x + 1

= 3x² + 3x - 9 - 3x² - 12x - 12

= -9x - 21

c) ( x + 3 )( x - 3 ) - ( x + 5 )( x - 1 ) - ( x - 4 )²

= x² - 9 - ( x² + 4x - 5 ) - ( x² - 8x + 16 )

= x² - 9 - x² - 4x + 5 - x² + 8x - 16

= -x² + 4x - 20

d) 2x( x + 1 )² - ( x - 1 )³ - ( x - 2 )( x² + 2x + 4 ) 

= 2x( x² + 2x + 1 ) - ( x³ - 3x² + 3x - 1 ) - ( x³ - 8 )

= 2x³ + 4x² + 2x - x³ + 3x² - 3x + 1 - x³ + 8

= 7x² - x + 9

e) ( x + 5 )( x - 5 )( x + 2 ) - ( x + 2 )³

= ( x² - 25 )( x + 2 ) - ( x³ + 6x² + 12x + 8 )

= x³ + 2x² - 25x - 50 - x³ - 6x² - 12x - 8

= -4x² - 37x - 58