... 6x 1 cos 8x 1 cos10x 1 cos12x2 2 2 2* - - + = - - + cos 6x cos 8x cos10x cos12x 2 cos7x cos x 2 cos11x cos x+ = + = ( ) ( ) x k2cos x 0lcos x cos 7x cos11x 0 x k, l, mcos 7x cos11x2mx9ộpờ= ... ứờ ỳở ỷ.( )( )2 4 2 4 4 2 1 1 1 sin 4x cos 4x 1 1 cos 4x cos 4x 2cos x cos 4x 1 02 2* - = - - = - - = ( )( ) 22 222t 1 N2t t 1 0 1 cos 4x 1 sin 4x 0 sin 4x 0t Lt cos 4x ... - 3x x 5x 3x 7x 5x 9x 7x 11 x 9x xsin sin sin sin sin sin sin sin sin sin sin2 2 2 2 2 2 2 2 2 2 2- + - + - + - + - = -.( ) 11 x 11 x 2msin 0 m x , m 11 , m2 2 11 p= = = p ạ ẻ Â.Bai giai...