Find the sum of the series `1+cos^2theta+cos^2thetacos2theta+cos^3 theta cos3theta+… to oo`

Find the sum of the series costheta.cos2theta+cos3theta.cos4theta+cos5theta.cos6theta+…to n terms

Find the sum of the series costheta- (cos2theta)/(2!)+(cos3theta)/(3!)-… to oo

Find the sum of n terms the series: 1/(costheta+cos3theta)+1/(costheta+cos5theta)+1/(costheta+cos7theta)+…

Solve cos theta+cos 2theta+cos 3theta=0 .

Prove: tan^2thetacos^2theta=1-cos^2theta

Sum to n terms the series costheta-sin2theta-cos3theta+sin4theta+cos5theta-……

The value of the expression cos^6theta+sin^6theta+3sin^2thetacos^2theta=

costheta+cos2theta+cos3theta+.......+cos(n-1)theta+cosntheta=

Solve cos theta+cos 3 theta-2 cos 2 theta=0 .

Prove that : cos^3 2theta+3cos2theta=4(cos^6theta-sin^6 theta)