l3a+1l+（2b-1)的2次方=0
l3a+1l=0,（2b-1)的2次方=0
a=-1/3,b=1/2
(3a^2-2b^3)/ab
=[3*(-1/3)^2-2*(1/2)^3]/[(-1/3)*1/2]
=[3*1/9-2*1/8]/(-1/6)
=[1/3-1/4]/(-1/6)
=[1/3-1/4]*6
=(4/12-3/12)*6
=1/12*6
=1/2