sin(45+α)=√5/5,则sinαcosα=

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  • 根据sin(45°-α)=5/13建立如图三角形关系.

    由上图求出 cos(45°-α)=12/13

    由和差角公式:

    sin(45°-α)=sin45°cosα-cos45°sinα = ( √2/2)(cosα-sinα) = 5/13

    cos(45°-α)=cos45°cosα +sin45°sinα=( √2/2)(cosα+sinα)=12/13

    cos(45°+α)=cos45°cosα -sin45°sinα=( √2/2)(cosα-sinα)=5/13

    ( √2/2)(cosα-sinα) = 5/13 (1)

    ( √2/2)(cosα+sinα)=12/13 (2)

    (1) (2)联立求解 sinα=(7√2)/26

    sinα/cos(45°+α)=((7√2)/26)/(13/5)=(7√2)/10