三角関数の加法定理を使います
▷sin(α+β)=sinαcosβ+cosαsinβ
三角関数の加法定理より、
sin(-θ)cos(θ+π/2)+sin(θ+π/2)cos(-θ)
=sin(-θ+θ+π/2)
=sin(π/2)
=1 ←答え
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