cos(α-π/2)=cosαcosπ/2+sinαsinπ/2 =sinα (cosπ/2=0,sinπ/2=1)
sin(α－π/2)=sinαcosπ/2-sinπ/2cosα=-cosα
1.证明 cos(α-π/2)=sinα sin（α－π/2)=﹣sinα
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