Question

tìm x

a) (x+1)^3 - (x-1)^3 - 6x^2 + 2x = 0

b) (2x - 1)^3 - 2x.(2x+1)^2=5x -20x^2

tui ms lớp 7 :<

Answer 1

`a)`

`->(x+1)^3-(x-1)^3-6x^2+2x=0`

`->x^3+3x^2+3x+1-(x^3-3x^2+3x-1)-6x^2+2x=0`

`->x^3+3x^2+3x+1-x^3+3x^2-3x+1-6x^2+2x=0`

`->(x^3-x^3)+(3x^2+3x^2-6x^2)+(3x-3x+2x)+(1+1)=0`
`->2x+2=0`

`->2x=-2`

`->x=(-2)/(2)`

`->x=-1`

Vậy `x=-1`

`b)`

`(2x-1)^3-2x(2x+1)^2=5x-20x^2`
`->(2x)^3-3.(2x)^2 . 1+3.2x.1^2-1^3-2x[(2x)^2+2.2x.1+1^2]=5x-20x^2`

`->8x^3-12x^2+6x-1-2x(4x^2+4x+1)=5x-20x^2`

`->8x^3-12x^2+6x-1-8x^3-8x-2x=5x-20x^2`

`->8x^3-12x^2+6x-1-8x^3-8x^2-2x-5x+20x^2=0`

`->(8x^3-8x^3)+(-12x^2-8x^2+20x^2)+(6x-2x-5x)-1=0`

`->-x-1=0`

`->-x=1`

`->x=-1`

Vậy `x=-1`