（x-y）^5÷（y-x）^4 =（x-y）^5÷（X-Y）^4 =X-Y
（x-y）^5÷（y-x）^4 =（x-y）^5÷（x-y）^4 =(x-y)^(5-4)=x-y
=（x-y）^5 ÷（x-y）^4=x-y
