5 S2 22 4SIN 2SIN SIN 2SIN 2
1,5 s2 22 4sin 2sin sin 2sin 2.3 x x x 2 Ta có : tan 7 tan 5 cot 3 6 2 6 Do đó : tan . tan 7 1 x x 6 3
2
2
52
5c x c x c xos os os3 31 c c c1 os2x 1 os(10 /3+2x) 1 os(10 /3-2x) 2 2 2c c3 os2x 2 cos(10 / 3) os2x 3Suy ra 3 4 9C