若cosa=-4/5,a是第三象限角,则sin(a+π/4)=

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cosa=-4/5,a是第三象限角
sina=-√1-cos^2a=-3/5
sin(a+π/4)=sinacosπ/4+cosasinπ/4
=√2/2(-4/5-3/5)
=-7√2/10
======以下答案可供参考======
供参考答案1:
sina=-3/5,sin(a+π/4)= sina*cosπ/4+cosa*sinπ/4=（-7/10）*根号2