`#tnvt`
Câu `6:`
`1/9x^2-1/64y^2`
`=(1/3x)^2-(1/8y)^2`
`=(1/3x-1/8y)(1/3x+1/8y)`
`=(x/3-y/8)(x/3+y/8)`
`->D`
Câu `7:`
`(3x-4y)^2`
`=(3x)^2-2.4y.3x+(4y)^2`
`=9x^2-24xy+16y^2`
`->A`
Câu `8:`
`25x^2-20xy+4y^2`
`=(5x)^2-2.5x.2y+(2y)^2`
`=(5x-2y)^2`
Câu `9:`
`A=(3x-1)^2-9x(x+1)`
`=(3x)^2-2.3x.1+1^2-9x^2-9x`
`=9x^2-6x+1-9x^2-9x`
`=-15x+1`
`=>A`
Câu `10:`
`(x-6)(x+6)-(x+3)^2=9`
`<=>x^2-6^2-x^2-2.3.x-3^2=9`
`<=>x^2-x^2-6x=9+9+36`
`<=>-6x=54`
`<=>x=-9`
`->A`
