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