CMR: $cosx.cos(60°-x). cos(60°+x)=\frac{1}{4}cos3x$

Áp dụng công thức : cos(a + b) = cos a cos b - sin a sin b

cos x * cos(60° - x) * cos(60° + x)= `1/4`cos3x

= `cos x * (cos 60° cos x - sin 60° sin x) * (cos 60° cos x + sin 60° sin x)``=1/4`cos3x

= `cos x * (cos² 60° cos² x - sin² 60° sin² x)`=`1/4`cos3x

= `cos x * (1/4 * cos² x - 3/4 * sin² x)`=`1/4`cos3x

= `1/4 * cos³ x - 3/4 * cos x * sin² x` = `1/4`cos3x

`=>` PT đúng