参照公式：a^2-b^2=(a+b)(a-b)（x+y)^2-(x-y)^2=（x+y+x-y）[(x+y)-(x-y)] =2x*2y =4xy
（x+y)^2-(x-y)^2=【（x+y）+(x-y）】*【（x+y）-(x-y）】=4xy
=4xy
2b
