(x+y)^2-(a+b)^2=(x+y+a+b+)(x+y-a-b)x²-4y²+x-2y=(x-2y)(x+2y)+(x-2y)=(x-2y)(x+2y+1) x^2(a+b-c)-2(c-b-a)=x²(a+b-c)+2(a+b-c)=(a+b-c)(x²+2)^什么意思??x^2是x²(x+y)^2是（x+y)²