计算cosa+cos(2π/3+a)+cos(4π/3+a)的值

问题描述:

计算cosa+cos(2π/3+a)+cos(4π/3+a)的值

cosa+cos(2π/3-a)+cos(4π/3+a)
=cosa+cos2π/3cosa+sin2π/3sina+cos2π/3cosa-sin2π/3sina
=cosa+2cos2π/3cosa
=cosa+2*(-1/2)cosa
=0