求大神帮解一道数学题

Question

求大神帮解一道数学题填空第三题

Answer 1

π/2<θ<π，则sinθ>0，cosθ<0
√(sin²θ-sin⁴θ)
=√[sin²θ(1-sin²θ)]
=√(sin²θcos²θ)
=|sinθcosθ|
=-sinθcosθ
=-½sin2θ