a+b=-c
ab=4/c
有因为a,b,c中两负一正
不妨设a,b0
则c>a,c>b
a,b是方程t^2+ct+4/c=0的两根
△=c^2-16/c≥0 c^3≥16
1/a+1/b=(a+b)/ab=-c^2/4
1/a+1/b+1/c=(4-c^3)/4c
4-c^30
1/a+1/b+1/c