化简（y-x）的3次方乘（x-y）的n次方+（x-y）的n+1次方乘（y-x）的平方

原式=[-(x-y)]^3(x-y)^n+(x-y)^(n+1)*[-(x-y)]^2
=-(x-y)^3(x-y)^n+(x-y)^(n+1)(x-y)^2
=-(x-y)^(n+3)+(x-y)^(n+3)
=0