（x+y+2)(x-y+2) =(x+2)^2-y^2=x^2+4x+4-y^2
x^5-x =x(x^2+1)(x+1)(x-1)
a²-2a-b²+1 =(a-1+b)(a-1-b)
.x²-y²-x+y=(x-y)(x+y-1)
1.（x+y+2)(x-y+2)
=(x+2-y)(x+2-y)
=(x+2)^2-y^2
=x^2+4x+4-y^2
分解因式
1.x^5-x ^2
=x^2(x^3-1)
=x^2(x-1)(x^2+x+1)
2.a²-2a-b²+1
=(a^2-2a+1)-b^2
=(a-1)^2-b^2
=(a-1+b)(a-1-b)
3.x²-y²-x+y
=(x+y)(x-y)-(x-y)
=(x-y)(x+y-1)
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