1.ab-a+b-1=(ab-a)+(b-1)=a(b-1)+(b-1)=(a+1)(b-1)2.3m+nx-3n-mx=(3m-3n)-(mx-nx)=3(m-n)-x(m-n)=(3-x)(m-n)3.题目有误,xy-xz-y^2+2yz-z^2=(xy-xz)-(y^2-2yz+z^2)=x(y-z)-(y-z)^2=(y-z)(x-y+z)