(1)x^2y^2-y^2=y^2(x^2-1)=y^2(x+1)(x-1)
(2)2x-4=2(x-2)
(3)x^2y+xy^2=xy(x+y)=0*(-7)=0
x^2+y^2=(x+y)^2-2xy=0-2*(-7)=14
(4)1+x^2不是完全平方
(5)(a-2)^2-6(2-a)=(a-2)(a-2+6)=(a-2)(a+4）=（-2-2）（-2+4）=-4*2=-8