把所给条件变形得到:2ac=b*(a+c)即(a+c)/ac=2/b即1/a+1/c=2/b证毕
ac-bc=ab-ac移项：2ac=bc+ab两边同除以abc: 2/b=1/a+1/c
由(a-b)/(b-c)=c/aab-ac=ac-bc2ac=ab+bcb=2ac/(a+c)1/a+1/c=(a+c)/ac2/b=(a+c)/ac1/a+1/c=2/b
