已知π/2
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![已知π/2](/uploads/image/z/1063871-71-1.jpg?t=%E5%B7%B2%E7%9F%A5%CF%80%2F2)
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已知π/2
已知π/2
已知π/2
π/2<β<α<3π/4,第二象限
π<α+β<3π/2第三象限
0<α-β<π/4第一象限
cos( α-β)=12/13==> sin( α-β)=5/13,
sin(α+β)=-3/5==>cos(α+β)=-4/5
sin2α
=sin(α+β + α-β)
=sin(α+β)cos(α-β)+cos(α+β)sin(α-β)
=-3/5*12/13 -4/5*5/13
=-56/65
cos2β
=cos(α+β - α-β)
=cos(α+β)cos(α-β)+sin(α+β)sin(α-β)
=-4/5*12/13 -3/5*5/13
=-63/65
=
抱歉,我的手机打不出希腊字母!sin=(-56/65) cos=(-63/65)