a^2b=-2
(a^2b)^2=a^4b^2=4
(a^2b)^3=a^6b^3=-8
ab=-2/a
ab^2=-2b/a
-ab(a^2b^5-ab^3-b)
=-a^3b^6+a^2b^4+ab^2
=8+4+ab^2
=12+ab^2
=12-2b/a
要是所求的那個式子括號中最后一個字母是a,這題就完美了.