cos 4x ·cos 8x -cos 5x ·cos 9x=0 <=> 1/2 ·(cos 12x -cos 4x) -1/2 ·(cos 14x -cos 4x)=0 <=>
1/2 ·(cos 12x -cos 4x -cos 14x +cos 4x)=0 <=> cos 12x -cos 14x=0 <=> -2sin 13x ·sin(-x)=0 <=> 2sin 13x ·sin x=0 => sin 13x ·sin x=0 <=> sin x=0 => x∈{0;180;360} => nu convine nici o varianta ;
sau sin 13x=0 <=> 13x∈{0;180;360} <=> x∈{0;180/13;360/13} => nu convine nici o varianta => nu exista solutii in (0,2) .
