1.=(a+b)x-(a+b)y-a(a-b)
=(a+b)(x-y)-a(a-b)
2.=a(a^2+4a+3)-(a^2-9)
=a(a+3)(a+1)-(a+3)(a-3)=(a+3)(a^2+3)
3.=2(x^2+2xy+y^2)-(x+y)-1
=2(x+y)^2-(x+y)-1
=(2x+2y+1)(x+y-1)
4.=1/25(5x-a+10)(5x-1)
5.=a^2(b-c)+b^2c-ab^2+ac^2-bc^2
=a^2(b-c)+b^2c-ab^2+ac^2-bc^2
=a^2(b-c)+bc(b-c)-a(b+c)(b-c)
=(b-c)(a^2+bc-ab-ac)
6.=(x^2-2x)^2-2(x^2-2x)-3
=(x^2-2x+1)(x^2-2x-3)
=(x-1)^2*(x-3)*(x+1)
7.=(ab-1)^2+(a+b-2)[a+b-2-2(ab-1)]
=(ab-1)^2-2(ab-1)(a+b-2)+(a+b-2)^2
=(ab-1-a-b+2)^2
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