Докажите тождество cosx+cos2x+cos6x+cos7x=4cosx/2*cos5x/2*cos4x

Question

Докажите тождество cosx+cos2x+cos6x+cos7x=4cosx/2*cos5x/2*cos4x

Answer 1

2cos3/2xcosx/2+2cosx/2cos13x/2=2cosx/2[cos3x/2+cos13x/2]==4cosx/2*cos5x/2*cos4x