Cos2a+cos14a+cos6a+cos10a
Cos2a+cos14a+cos6a+cos10a=4cos8a*cos4a*cos2a cos2a+cos14a=2cos \frac{2a+14a}{2} cos \frac{14a-2a}{2} cos6a+cos10a=2cos \frac{6a+10a}{2} cos \frac{10a-6a}{2} 2cos8a*cos6a+2cos8a*cos2a=2cos8a*(cos6a+cos2a)=2cos8a*2cos4a*cos2a.