任意△ABC,求证:1/2(1/a+1/b+1/c)
问题描述：
任意△ABC,求证:1/2(1/a+1/b+1/c)
答
1）题是1/2*(1/a+1/b+1/c)=2ab,b^2+c^2>=2bc,c^2+a^2>=2ca a^2+b^2+c^2>=ab+bc+ca (cosA/a+cosB/b+cosC/c)-0.5(1/a+1/b+1/c) =(a^2+b^2+c^2)/(2abc)-(ab+bc+ca)/(2abc)>=0 cosA/a+cosB/b+cosC/c≥0.5(1/a+1/b+1/c) ...