cotA=cosA/sinA=cosA/a *2R
同理,cotA+cotB+cotC
=2R(cosA/a+cosB/b+cosC/c)
=2R[ (b^2+c^2-a^2)/2abc+(a^2+c^2-b^2)/2abc+(a^2+b^2-c^2)/2abc ]
=R(a^2+b^2+c^2)/abc
有问题追问