(3a^2-ab)/(3a^2+5ab-2b^2)
=a(3a-b)/(3a-b)(a+2b)
=a/(a+2b)
=(1/2)/(1/2+2/3)
=3/7
将a=1/2,b=1/3代入原式
得：
3a2-ab/3a2+5ab-2b2
=3(1/2)2-(1/2*1/3)/[3(1/2)2]+5*1/2*1/3-2*(1/3)2
=3/4-2/9+5/6-2/9
=27/36-16/36+30/36
=41/36
直接代入 3*1/4-(1/3)/(3*1/2)+5(1/2)(1/3)-2*1/9=3/4-2/9+5/6-2/9=73/36
题目是（3a^2-ab)/(3a^2+5ab-2b^2),还是3a^2-ab/3a^2+5ab-2b^2
10
3/7 7分之3
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