7x的平方-3y+xy-21x
=7x^2+xy-3y-21x
=x(7x+y)-3(7x+y)
=(7x+y)(x-3)
1.x的平方-2xy+y的平方-9
=(x-y)^2-9
=(x-y+3)(x-y-3)
2.a的平方-b的平方-2b-1
=a^2-(b^2+2b+1)
=a^2-(b+1)^2
=(a+b+1)(a-b-1)
3.a的平方-b的平方-c的平方+2bc
=a^2-(b^2-2bc+c^2)
=a^2-(b-c)^2
=(a+b-c)(a-b+c)
4.x的平方-6x+9-y的平方
=(x-3)^2-y^2
=(x-3+y)(x-3-y)
=(x+y-3)(x-y-3)
5.y的四次方-2y的平方+1
=(y^2-1)^2
=(y+1)^2(y-1)^2
6.1-x的平方+4y(x-y)
=1-x^2+4xy-4y^2
=1-(x^2-4xy+4y^2)
=1-(x-2y)^2
=(1+x-2y)(1-x+2y)