a的平方-b的平方+2(ax-ay)+(x的平方-y的平方)
=a²-b²+2(ax-ay)+(x²-y²)
=a²-b²+2ax-2ay+x²-y²
=(a²+2ax+x²)-(b²+2ay+y²)
=(a+x)²-(b+y)²
=(a+b+x+y)(a-b+x-y)