Giải thích các bước giải :
`5)x^2-3x+2=(x^2-x)-(2x-2)=x(x-1)-2(x-1)=(x-1)(x-2)`
`6)x^2+8x-9=(x^2+9x)-(x+9)=x(x+9)-(x+9)=(x+9)(x-1)`
`7)x^2-10x+16=(x^2-2x)-(8x-16)=x(x-2)-8(x-2)=(x-2)(x-8)`
`8)3x^2-6x-9=3(x^2-2x-3)=3[(x^2+x)-(3x+3)]=3[x(x+1)-3(x+1)]=3(x+1)(x-3)`
`9)2x^2+18x+40=2(x^2+9x+20)=2[(x^2+4x)+(5x+20)]=2[x(x+4)+5(x+4)]=2(x+4)(x+5)`
`10)3x^2-18x+42=3(x^2-6x+14)`
