x的负一次方-y的负一次方x的负二次方-y的负二次方
=(1/x-1/y)/(1/x^2-1/y^2)
=(1/x-1/y)/(1/x+1/y)(1/x-1/y)
=1/(1/x+1/y)
=1/[(x+y)/xy]
=xy/(x+y)
（4ab）四次方÷（a²b8）的负三次方
=4^4/a^4b^4÷a^(-6)b^(-3)/8^(-3)
=256/a^4b^4÷512/a^(6)b^(3)
=256/a^4b^4*a^6b^3/512
=a^2/(2b)
(-a^2)^3*(-a^3)^(-2)
=-a^6*a^(-6)
=-a^(6-6)
=-1