已知a=3/4,b=-2/3
(3a^2-ab)/(9a^2-6ab+b^2)
=(3*9/4+17/12)/(9*9/4-4/3)
=(27/4+17/12)/(81/4-4/3)
=(81/12+17/12)/(243/12-16/12)
=(98/12)/(227/12)
=98/227