प्रश्न से `cos theta+cos2 theta+cos 3 theta=0`
या `cos 2 theta+2cos2 theta. cos theta=0` या `cos 2 theta(1+2 costheta)=0`
`:. cos2theta=0` या `1+2cos theta=0`
जब `cos 2 theta=0` तो `2 theta=(2n+1)(pi)/(2) rArr theta=(2n+1)(pi)/(4)`
जब `1+2 cos theta =0` तो `cos theta=-(1)/(2)="cos"(2pi)/(3), " ":. theta=2npipm(2pi)/(3).n in Z`
अतः `theta=(2n+1)(pi)/(4),2npipm(2pi)/(3), n in Z`
