The correct option is D tan(θ2)−tan(θ64)
Given:
5∑n=1tan(θ2n+1)sec(θ2n)
Assuming α=θ2n, so
5∑n=1tan(α2)sec(α)=5∑n=1sinα2cosαcosα2=5∑n=1sin(α−α2)cosαcosα2=5∑n=1sinαcosα2−cosαsinα2cosαcosα2=5∑n=1tanα−tanα2=5∑n=1tan(θ2n)−tan(θ2n+1)=tan(θ21)−tan(θ22) +tan(θ22)−tan(θ23) +tan(θ23)−tan(θ24) +tan(θ24)−tan(θ25) +tan(θ25)−tan(θ26)=tan(θ21)−tan(θ26)=tan(θ2)−tan(θ64)