un=sinnθ+cosnθ
Now u5−u7u3−u5
=sin5θ+cos5θ−sin7θ−cos7θcos3θ+sin3θ−sin5θ−cos5θ
=sin5θ+cos5θ−sin5θ(1−cos2θ)−cos5θ(1−sin2θ)cos3θ+sin3θ−sin3θ(1−cos2θ)−cos3θ(1−sin2θ)
=sin5θcos2θ+cos5θsin2θsin3θcos2θ+cos3θsin3θ
=(sin3θ+cos3θ)(sin2θcos2θ)(sinθ+cosθ)(sin2θcos2θ)=u3u1