Bài 1 :
a,`(x+y)^(3)-(x-y)^3`
`=(x+y-x+y)[(x+y)^(2)+(x+y)(x-y)+(x-y)^2]`
`=(x+y-x+y)(x^(2)+2xy+y^(2) + x^(2)-y^(2) + x^(2)-2xy+y^2))`
`=2y(3x^(2)+y^2)`
`b,16x^(2)-9.(x+y)^2`
`(2x)^(2)-[3(x+y)^2]`
`=[2x-3.(x+y)][2x+3.(x+y)]`
`=(2x-3x+3y)(2x+3x+3y)`
`=(-x+3y)(5x+3y)`
`c,36x^(2)-z^(2)-9y^(2)-6zy^(2)`
`=36x^(2)-z^(2)-(3y)^(2)-6zy^(2)`
`=(6x)^(2)-(z^(2)+6zy^(2)+(3y)^2)`
`=(6x)^(2)-(z+3y)`
`=(6x-z-3y)(6x+z+3y)`
`d,x^(4)-25x^(2)+20x-4`
`=x^(4)-(5x)^2+20x-4`
`=(x^2)^(2)-(5x^(2)-20x + 2^2)`
`=(x^2)^(2)-(5x-2)^2`
`(x^(2)-5x+2)(x^(2)+5x-2)`
