COT2 2 1 1 A B CCOT TAN A B C2 2 COT COT COT 2 2 21 A B C A B COT
3.b. A B cot .cot2 2 1 1 A B Ccot tan A B C2 2 cot cot cot 2 2 21 A B C A B cot .cot 1 cot cot cot Ta có: 2 2 2 2 2A B C A B C dfcmcot cot cot cot .cot .cot2 2 2 2 2 2