用倒数:(a+b)/ab=1/a+1/b=3(1),(b+c)/bc=1/b+1/c=4(2),(c+a)/ac=1/c+1/a=5(3)(3)-(1)得:1/c-1/b=2(4)(2)+(4)得:2/c=6,c=1/3(2)-(4)得:2/b=2,b=1将b=1代入(1)得:a=1/2