1/a+1/2b=32a-5ab+4b/4ab-3a-6b分数式的分子分母上下同除以4ab商不变.（易知4ab不为0)=(1/2b-5/4+1/a)/(1-3/4b-3/2a)=(1/2b+1/a-5/4)/(1-3/2(1/a+1/2b)) 代入已知 1/a+1/2b=3=(3-5/4)/(1-(3/2*3）)=(7/4)/1-9/2)=7/4/(-7/2)=-1/2
