(3a^2-ab)/(3a^2+5ab-2b^2)=a(3a-b)/[(3a-b)(a+2b)]=a/(a+2b)=1/2/(1/2+1/3)=1/2/(5/6)=3/5
原式=a(3a-b)(3a-b)(a+2b)=a/(a+2b)=(1/2)/(1/2+2/3)=3/7
