(x-2y+z)(-x+2y+z)=(z+x-2y)(z-(x-2y))=z^2-(x-2y)^2=z^2-x^2+4xy-4y^2（2x+3y-1）（2x-3y+1）=[2x+(3y-1)][2x-(3y-1)]=(2x)^2-(3y-1)^2=4x^2-(9y^2-6y+1)=4x^2-9y^2+6y-1