若sina=-4/5,a是第三象限的角,则sin(a+π/4)=A.-7√2/10 B.7√2/10 C.-√2/10 D.√2/10

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若sina=-4/5,a是第三象限的角,则sin(a+π/4)=A.-7√2/10 B.7√2/10 C.-√2/10 D.√2/10
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若sina=-4/5,a是第三象限的角,则sin(a+π/4)=A.-7√2/10 B.7√2/10 C.-√2/10 D.√2/10
若sina=-4/5,a是第三象限的角,则sin(a+π/4)=
A.-7√2/10 B.7√2/10 C.-√2/10 D.√2/10

若sina=-4/5,a是第三象限的角,则sin(a+π/4)=A.-7√2/10 B.7√2/10 C.-√2/10 D.√2/10
若sina=-4/5,a是第三象限的角,则cosa=-3/5
sin(a+π/4)=sinacosπ/4+cosasinπ/4=-(4/5+3/5)√2/2=-7√2/10