Bài 3:
`a)9-4x²`
`=3²-(2x)²`
`=(3+2x)(3-2x)`
`b)4y^4-25x^6`
`=(2y^2)^2-(5x^3)^2`
`=(2y^2+5x^3)(2y^2-5x^3)`
`c)8-27x^3`
`=2^3-(3x)^3`
`=(2-3x)[2^2+2.3x+(3x)^2]`
`=(2-3x)(4+6x+9x²)`
`d1)(3x+1)²-16`
`=(3x+1)²-4²`
`=(3x+1+4)(3x+1-4)`
`=(3x+5)(3x-3)`
`=3(3x+5)(x-1)`
`d2)25.(x+3)²-9.(7-2x)²`
`=[5(x+3)]²-[3(7-2x)]²`
`=[5(x+3)+3(7-2x)][5(x+3)-3(7-2x)]`
`=(5x+15+21-6x)(5x+15-21+6x)`
`=(36-x)(11x-6)`
`e)125+(2x+1)³`
`=5³+(2x+1)³`
`=(5+2x+1)[5²-5.(2x+1)+(2x+1)²]`
`=(2x+6)(25-10x-5+4x²+4x+1)`
`=2(x+3)(4x²-6x+21)`
Bài 4:
`a)x²-6x+9`
`=x²-2.x.3+3²`
`=(x-3)²`
`b)4x²+20xy+25y²`
`=(2x)²+2.2x.5y+(5y)²`
`=(2x+5y)²`
`c)`Sửa đề:`-9x²+6xy-1`
`→-9x²+6x-1`
`-9x²+6x-1`
`=-(9x²-6x+1)`
`=-[(3x)²-2.3x.1+1²]`
`=-(3x-1)²`
`d1)x³-3x²+3x-1`
`=x³-3.x².1+3.x.1²-1³`
`=(x-1)³`
`d2)x³+6x²+12x+8`
`=x³+3.x².2+3.x.2²+2³`
`=(x+2)³`