13.(2023•永州三模)已知cosθ−cos2θ−1=0\cos \theta -\cos 2\theta -1=0cosθ−cos2θ−1=0,θ∈(0,π2)\theta \in ({0,\frac{\pi }{2}})θ∈(0,2π),则cosθ=(\cos \theta =(cosθ=( )))