sec4θ−sec2θ is equal to
tan2θ−tan4θ
tan4θ−tan2θ
tan4θ+tan2θ
tanθ+tan4θ
sec4θ−sec2θ=sec2θ(sec2θ−1) =(1+tan2θ)(tan2θ)[∵ sec2θ=1+tan2θ] =tan4θ+tan2θ