1,X=a/(b+c)=b/(a+C)=C/(a+b)=(a+b+c)/(2a+2b+2c)=1/2
2,a/b=(a-c)/(c-b)= (2a-c)/c= c/(2b-c)
2ab=ab+bc
1/a+1/b=2/c
错了,第一道题还少了a+b+c=0的情况,所以当a+b+c=0时X=a/-a=-1