4（x-y）的n加1次方-（x+y）²×（x-y）的n减1次方
=（x-y）^(n-1)[4(x-y)²-(x-y)²]
=(x-y)^(n-1)【2(x-y)+(x-y)】【2(x-y)-(x-y)】
=(x-y)^(n-1)【3x-3y】【x-y】
=3(x-y)^(n-1)(x-y)²
=3(x-y)^(n+1)