（ab分之4的4次方）除以（8分之a平方b）的负三次方
=(4/ab)^4 /(a^2b/8)^(-3)
=(4/ab)^4 *(a^2b/8)^3
=4^4/a^4b^4 *(a^6b^3/8^3)
=2^8/a^4b^4 *(a^6b^3/2^9)
=a^2/2b