ab/a+b=1/4(a+b)/ab=41/a + 1/b=4bc/b+c=1/3(b+c)/bc=31/b + 1/c=3ca/c+a=1/2(c+a)/ca=21/a + 1/c=2三个式子相加2(1/a + 1/b + 1/c)=91/a + 1/b + 1/c=9/2(ab+bc+ca)/abc=9/2abc/(ab+bc+ca)=2/9