Gọi $x =\tan A,y= \tan B, z = \tan C$Ta có $\tan A. \tan B. \tan C=\tan A+ \tan B+ \tan C$$\Leftrightarrow \tan A(\tan B. \tan C-1)= \tan B + \tan C$$\Leftrightarrow \frac{\tan B+ \tan C}{1-\tan B. \tan C}=-tan A$$\Leftrightarrow \tan (B+C)=\tan (\pi -A)$$\Leftrightarrow A+B+C= \pi$$\Leftrightarrow T = \cot A+ \cot B + \cot C \ge \sqrt 3 $
Gọi $x =\tan A,y= \tan B, z = \tan C$Ta có $\tan A. \tan B. \tan C=\tan A+ \tan B+ \tan C$$\Leftrightarrow \tan A(\tan B. \tan C-1)= \tan B + \tan C$$\Leftrightarrow \frac{\tan B+ \tan C}{1-\tan B. \tan C}=-tan A$$\Leftrightarrow \tan (A+B)=\tan (\pi -C)$$\Leftrightarrow A+B+C= \pi$$\Leftrightarrow T = \cot A+ \cot B + \cot C \ge \sqrt 3 $
Gọi $x =\tan A,y= \tan B, z = \tan C$Ta có $\tan A. \tan B. \tan C=\tan A+ \tan B+ \tan C$$\Leftrightarrow \tan A(\tan B. \tan C-1)= \tan B + \tan C$$\Leftrightarrow \frac{\tan B+ \tan C}{1-\tan B. \tan C}=-tan A$$\Leftrightarrow \tan (B
+C)=\tan (\pi -
A)$$\Leftrightarrow A+B+C= \pi$$\Leftrightarrow T = \cot A+ \cot B + \cot C \ge \sqrt 3 $