Cho cos anpha/2 = 0. Khi đó cos anpha + cos 2anpha + cos 4anpha + cos7anpha bằng
Giải thích
cosα=2.cos2α2-1=2.0-1=-1cos2α=2cos2α-1=2.(-1)2-1=1cos4α=2cos22α-1=2.12-1=1cos7α=cosa+6a=cosα.cos6α-sinαsin6α=-cos(3.2α)=-4cos32α+3cos2α=-4+3=-1cosα+cos2α+cos4α+cos7α=-1+1+1+(-1)=0