... ()()()() 11 1 1 1 cos 6x 1 cos 8x 1 cos10x 1 cos12x22 2 2−−+=− −+ ⇔ cos 6x cos8x cos10x cos12x+= +⇔ 2cos7xcosx 2cos11xcosx=⇔ ( )2cos x cos7x cos11x 0−= ⇔ cos x 0 cos7x cos11x=∨ ... 222cot g b1 cotg c1cotg bcotg bcotg c=+−+− 1 =− (1) * ()221cosa1cosa 1 2sina sin a⎡⎤−+−⎢⎥⎢⎥⎣⎦ ()221cosa1cosa 1 2sina 1 cos a⎡⎤−+=−⎢⎥−⎢⎥⎣⎦ 1cosa 1cosa 1 2sina 1 cosa+−⎡⎤=−⎢⎥+⎣⎦ ... 1 1221tgxtgxB 1 tgx1 tgx11tgx 1 tgx++⇔= + = +−−−− ( )21tgx1tgxB1tgx 1 tgx 1 −−−⇔= = =−−− (không phụ thuộc vào x) Bài 4: Chứng minh ()222222221cosa1cosa...