第一个(a+1)(a+2)
第二个=x^2+2xy-4x^2+y^2=y^2+2xy-3x^2=(y-x)(y+3x)
第一题:a^2+3a+2 第二题:-3x^2+2xy+y^2
(a+1)^2+a+1
=(a+1)(a+1+1)
=(a+1)(a+2)
(x+y)^2-4x^2
=[(x+y)-2x][(x+y)+2x]
=(y-x)(3x+y)
(a+1)^2+a+1
=(a+1)(a+1+1)=(a+1)(a+2)
(x+y)^2-4x^2
=(x+y)^2-(2x)^2
=(x+y+2x)(x+y-2x)
=(3x+y)(y-x)
a^2+2a+1+a+1=a^2+3a+2
x^2+2xy+y^2-4x^2=-2x^2+2xy+y^2