`#AkaShi`
Bài `2` :
`x^2+4x+4-9y^2`
`=(x^2+4x+1)-(3y)^2`
`=(x+2)^2-(3y)^2`
`=(x+2-3y)(x+2+3y)`
$\text{___________________________}$
`y²-x²+6x-9`
`=y²-(x²-6x+9)`
`=y²-(x-3)^2`
`=(y-x+3)(y+x-3)`
$\text{___________________________}$
`9-x^2+2xy-y^2`
`=9-(x²-2xy+y²)`
`=3²-(x-y)²`
`=(3-x+y)(3+x-y)`
$\text{___________________________}$
Áp dụng HĐT:
`(A+B)^2=A^2+2AB+B^2`
`(A-B)^2=A^2-2AB+B^2`
`A^2-B^2=(A+B)(A-B)`
