设0≤θ设0≤θ
设0≤θ0≤θ
∵sinθ<0∴π<θ<2π∵cos2θ<0∴1-2sin²θ<0∴sinθ>√2/2,或sinθ<-√2/2∴π/4<θ<3π/4或5π/4<θ<7π/4所以5π/4<θ<7π/4