Cos5x +cosx-cos2x=0
2cos(5x+x)/2·cos(5x-x)/2-cos2x=0
2cos3x·cos2x-cos2x=0
cos2x(2cos3x-1)=0
cos2x=0 2cos3x=1
2x=π/2+πn n∈Z cos3x=1/2
x=π/4+πn/2 n∈Z 3x=+-π/3+2πn n∈Z
x=+-π/9+2/9·πn
отрезок -π до π
4 корня первой серии: -3π/4; -π/4;π/4; 3π/4
Х=π/9+2πn/9 n∈Z
20° ;20+40=60;100; 140;180;-20; -60; -100; -140; -180 корней10
или π/9; 3π/9=π/3; 5π/9;7π/9;π;-π/9; -π/3;-5π/9; -7π/9;-π
х=-π/9+2πn n∈Z
Получаем те же самые 10 корней