(x-y)的七次方÷(y-x)的三次方÷(y-x)的三次方
=(x-y)的七次方÷[(y-x)的三次方乘(y-x)的三次方]
=(x-y)的七次方÷(y-x)的6次方
=(x-y)的七次方÷(x-y)的6次方
=x-y