Đáp án:
`1,`
`a) (x-3)(x+3)+(x-3)^2=(x-3)(x+3+x-3)=2x(x-3)`
`b) x^2-2xy+x-2y=x(x-2y)+(x-2y)=(x-2y)(x+1)`
`c) x^2-2x+1-y^2=(x-1)^2-y^2=(x-y-1)(x+y-1)`
`d) xy+y^2=y(x+y)`
`e) x^2+4xy+4y^2-25=(x+2y)^2-5^2=(x+2y-5)(x+2y+5)`
`2, x^2+y^2-4x+6y+13=0`
`<=> (x^2-4x+4)+(y^2+6y+9)=0`
`<=> (x-2)^2+(y+3)^2=0`
do `(x-2)^2>=0; (y+3)^2>=0` với `∀x;y`
`=> (x-2)^2=0; (y+3)^2=0`
`<=>x-2=0; y+3=0`
`<=> x=2; y=-3`
`3,`
`a) (-9x^5y^2):(-6xy^2)`
`=3/2x^4`
`b) (15x^3-10x^2-5x):(-5x)`
`=-3x^2+2x+1`