(x-y)的3次方×(y-x)的4次方÷(y-x)的3次方÷(x-y)
=(x-y)的3次方×(x-y)的4次方÷[-(x-y)的3次方]÷(x-y)
=-(x-y)的(3+4-3-1)
=-(x-y)的3次方