Question

cos2x+cos4x+cos6x= ???????

Answer 1

cos2x+cos4x+cos6x= cos12x

Answer 2

cos 2x + cos 4x + cos 6x = 0 

cos 4x + (cos 2x + cos 6x) = 0 

cos 4x + (2 cos 4x cos 2x) = 0 

(1 + 2 cos 2x) cos 4x = 0 

(1 + 2 cos 2x) = 0 

cos 2x = -½ 

2x = 120° 

x = 60° 

sau

2x = 240° 

x = 120° 




sau

cos 4x = 0 

4x = 90° 

x = 22.5° 

sau

4x = 270° 

x = 67.5° 



x = 22.5° , 60° , 67.5° , 120°