切线法下证:a^2-3a+2(a)^0.5>=0,设t=(a)^0.5即证明t*(t-1)^2*(t+2)>=0,显然.故a^2+2(a)^0.5>=3a,b^2+2(b)^0.5>=3b,c^2+2(c)^0.5>=3c.相加得2((a)^0.5+(b)^0.5+(c)^0.5)>=9-(a^2+b^2+c^2)即根号a+根号b+根号c≥ab+bc+...