1/a+1/b=2即a+b=2ab,2.5a+2.5b=5ab
代入后式得:
3a-2ab+3b=3a-a-b+3b=2a+2b
-4a+5ab-4b=-4a+2.5a+2.5b-4b=-1.5a-1.5b
上下相除得 -4/3