Xét trong 1 tam giác:

\(\tan A+\tan B+\tan C=\tan\left(A+B\right).\left(1-\tan A.\tan B\right)+\tan C\)

\(=\tan\left(\pi-C\right)\left(1-\tan A.\tan B\right)+\tan C\)

\(=\tan A.\tan B.\tan C\)

☕ Quay lại bài toán, cần chứng minh \(\dfrac{1}{\tan A}+\dfrac{1}{\tan B}+\dfrac{1}{\tan C}\ge\sqrt{3}\)

Theo AM-GM:

\(VT^2\ge3\left(\dfrac{1}{\tan A.\tan B}+\dfrac{1}{\tan B.\tan C}+\dfrac{1}{\tan C.\tan A}\right)\)

\(=\dfrac{3\left(\tan A+\tan B+\tan C\right)}{\tan A.\tan B.\tan C}=3\). Suy ra đpcm

Chỉ đúng với điều kiện A, B, C là 3 góc trong tam giác \(\Rightarrow A+B+C=\pi\)

Đặt \(\frac{A}{2}=x\) , \(\frac{B}{2}=y\); \(\frac{C}{2}=z\) \(\Rightarrow x+y+z=\frac{\pi}{2}\Rightarrow\left\{{}\begin{matrix}x+y=\frac{\pi}{2}-z\\z=\frac{\pi}{2}-\left(x+y\right)\end{matrix}\right.\)

\(cot\frac{A}{2}+cot\frac{B}{2}+cot\frac{C}{2}=cotx+coty+cotz=\frac{cosx}{sinx}+\frac{cosy}{siny}+\frac{cosz}{sinz}\)

\(=\frac{cosx.siny+cosy.sinx}{sinx.siny}+\frac{cosz}{sinz}=\frac{sin\left(x+y\right)}{sinx.siny}+\frac{cosz}{sinz}\)

\(=\frac{sin\left(\frac{\pi}{2}-z\right)}{sinx.siny}+\frac{cosz}{sinz}=\frac{cosz}{sinx.siny}+\frac{cosz}{sinz}=cosz\left(\frac{1}{sinx.siny}+\frac{1}{sinz}\right)\)

\(=\frac{cosz}{sinx.siny.sinz}\left(sinz+sinx.siny\right)=\frac{cosz}{sinx.siny.sinz}\left(sin\left(\frac{\pi}{2}-\left(x+y\right)\right)+sinxsiny\right)\)

\(=\frac{cosz}{sinx.siny.sinz}\left(cos\left(x+y\right)+sinx.siny\right)\)

\(=\frac{cosz}{sinx.siny.sinz}\left(cosx.cosy-sinx.siny+sinx.siny\right)\)

\(=\frac{cosx.cosy.cosz}{sinx.siny.sinz}=cotx.coty.cotz=cot\frac{A}{2}.cot\frac{B}{2}.cot\frac{C}{2}\)