解
(y-x)³(x-y)^n+2(x-y)^(n+1)(y-x)²
=-(x-y)³(x-y)^n+2(x-y)^(n+1)(x-y)²
=-(x-y)^(n+3)+2(x-y)^(n+3)
=(x-y)^(n+3)