Đáp án:
`6. (x-y+4)^2-(2x+3y-1)^2`
`= [(x-y+4)+(2x+3y-1)][(x-y+4)-(2x+3y-1)]`
`= [x-y+4+2x+3y-1][x-y+4-2x-3y+1]`
`= [3x+2y+3][-x-4y+5]`
`= -[3x+2y+3][x+4y-5]`
`7. 100x^2-(x^2+25)^2`
`= (10x)^2-(x^2+25)^2`
`= [10x+(x^2+25)][10x-(x^2+25)]`
`= [10x+x^2+25][10x-x^2-25]`
`= -(x+5)^2(x-5)^2`
`8. (x-y+5)^2-2(x-y+5)+1`
`= (x-y+5)^2-2xx(x-y+5)xx1+1^2`
`= [(x-y+5)-1]^2`
`= [x-y+5-1]^2`
`= [x-y+4]^2`
`9. (x^2+4y^2-5)^2-16(x^2y^2+2xy+1)`
`= (x^2+4y^2-5)^2-[4(xy+1)]^2`
`= [(x^2+4y^2-5)+4(xy+1)][(x^2+4y^2-5)-4(xy+1)]`
`= [x^2+4y^2-5+4xy+4][x^2+4y^2-5-4xy-4]`
`= [x^2+4y^2-1+4xy][x^2+4y^2-9-4xy]`
`= [(x-2y)^2-9][(x+2y)^2-1]`
`= (x-2y+3)(x-2y-3)(x+2y+1)(x+2y-1)`