Giải thích các bước giải:
`VD4:`
`a,x^2+6x+9`
`=x^2+2.x.3+3^2`
`=(x+3)^2`
`b,9x^2-6x+1`
`=(3x)^2-2.3x.1+1^2`
`=(3x-1)^2`
`c,x^2y^2+xy+1/4`
`=(xy)^2+2.xy. 1/2+(1/2)^2`
`=(xy+1/2)^2`
`d,(x-y)^2+6(x-y)+9`
`=(x-y)^2+2.3(x-y)+3^2`
`=(x-y+3)^2`
`VD5:`
`a, x^2+6x+9`
`=x^2+2.x.3+3^2`
`=(x+3)^2`
`b,4x^2-4x+1`
`=(2x)^2-2.2x.1+1^2`
`=(2x-1)^2`
`c,9x^2-12xy+4y^2`
`=(3x)^2-2.3x.2y+(2y)^2`
`=(3x-2y)^2`
`d,(x-y/3)(x+y/3)`
`=x^2-(y/3)^2`
`=x^2-y^2/9`
* Lưu ý: Áp dụng các hằng đẳng thức sau:
`1) (A+B)^2=A^2+2AB+B^2`
`2) (A-B)^2=A^2-2AB+B^2`
`3) A^2-B^2=(A-B)(A+B)`
