1.(a+b)+(a^2-b^2)=(a+b)+(a+b)(a-b)=(a+b)(1+a-b)
2. (x-y)+(x^2-y^2)=(x-y)+(x-y)(x+y)=(x-y)(1+x+y)
3. (2-x)-(4-x^2)=(2-x)-(2-x)(2+x)=(2-x)(1-2-x)=(2-x)(-1-x)=-(2-x)(1+x)
4. (y-1)-(y^2-1)=(y-1)-(y-1)(y+1)=(y-1)(1-y-1)=(y-1)(-y)=-y(y-1)
1.(a+b)+(a^2-b^2)=(a+b)+(a+b)(a-b)=(a+b)(1+a-b)
2. (x-y)+(x^2-y^2)=(x-y)+(x-y)(x+y)=(x-y)(1+x+y)
3. (2-x)-(4-x^2)=(2-x)-(2-x)(2+x)=(2-x)(1-2-x)=(2-x)(-1-x)=-(2-x)(1+x)
4. (y-1)-(y^2-1)=(y-1)-(y-1)(y+1)=(y-1)(1-y-1)=(y-1)(-y)=-y(y-1)