Cos0,7xcos1,3x-sin0,7xsin1,3x=sin7xcos9x-sin9xcos7x x принадлежит [-п; п]

Cos0,7xcos1,3x-sin0,7xsin1,3x=sin7xcos9x-sin9xcos7x x принадлежит [-п;п]cos0,7x*cos1,3x -sin0,7x*sin1,3x = sin7x*cos9x - sin9x*cos7x ;
cos(1,3x - 0,7x)=sin(7x - 9x) ;
cos0,6x +sin2x=0 ;
cos0,6x +cos(π/2 -2x) =0 ;
2cos(0,7x -π/4)*cos( 1,3x -π/4)=0 ;
a) cos(0,7x -π/4) =0  ; 
0,7x -π/4 =π/2 +π*K
x = 15π/14 + 10π/7*k;
b) cos( 1,3x -π/4)=0 ;
1,3x₂ -π/4 =π/2 +π*n ;
x₂ = 15π/26 +10π/13*n;

x ₁= -  5π/14 ,  при k = -1
x₂ = -  5π/26 , при  n= -1

ответ : { -  5π/26 ; -  5π/14 }