Question

CHO TAM GIÁC NHỌN ABC. CHỨNG MINH RẰNG \(\cot A+\cot B+\cot C\ge\)\(\sqrt{3}\)

Answer 1

Đùa tí :v, Ta có:

\(tanA+tanB+tanC=tanAtanBtanC\)

Vi` vay \(cotAcotB+cotBcotC+cotCcotA=1\)

Va` \(\left(cotA-cotB\right)^2+\left(cotB-cotC\right)^2+\left(cotC-cotA\right)^2\ge0\)

Vi` vay \(cot^2A+cot^2B+cot^2C\ge1\)

Then \(\left(cotA+cotB+cotC\right)^2=cot^2A+cot^2B+cot^2C+2\left(cotAcotB+cotBcotC+cotCcotA\right)\ge3\)

Nen \(cotA+cotB+cotC\ge\sqrt{3}\)

Xay ra khi \(cotA=cotB=cotC\)

Answer 2

\(cotx\) là hàm lồi trên \(\left(0;\frac{\pi}{2}\right)\) và \(A,B,C\in\left(0;\frac{\pi}{2}\right)\)

Thì theo BĐT Jensen ta có: 

\(cotA+cotB+cotC\ge3cot\left(\frac{A+B+C}{3}\right)=\sqrt{3}\)

Xong :v