已知sin(a+60)+sina=-(4根3)/5,-90
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已知sin(a+60)+sina=-(4根3)/5,-90
sin(a+60)+sina
=sinacos60+cosasin60+sina
=1/2sina+√3/2cosa+sina
=3/2sina+√3/2cosa
=√3(√3/2sina+1/2cosa)
=√3(sina60sina+cos60cosa)
=√3(cos60cosa+sina60sina)
=√3cos(60-a)
=√3cos(180-120-a)
=√3cos(180-120-a)
=-√3cos(120+a)
-4√3/5=-√3cos(120+a)
4/5=cos(120+a)
cos(120+a)=4/5
=sinacos60+cosasin60+sina
=1/2sina+√3/2cosa+sina
=3/2sina+√3/2cosa
=√3(√3/2sina+1/2cosa)
=√3(sina60sina+cos60cosa)
=√3(cos60cosa+sina60sina)
=√3cos(60-a)
=√3cos(180-120-a)
=√3cos(180-120-a)
=-√3cos(120+a)
-4√3/5=-√3cos(120+a)
4/5=cos(120+a)
cos(120+a)=4/5
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