sina+sinb=根号3(cosb-cosa),a.b属于第一象线角.则sin3a+sin3b=?

sina+sinb=根号3(cosb-cosa)
左右平方
2sinasinb+1=根号3+2cosbcosa
2sinasinb-2cosacosb=根号3-1
sin(a-b)=(根号3-1)/2
cos(a-b)=根号(1-sin(a-b)^2)=+ 你自己算
sin3α=sin(2α+α)
=sin2α·cosα+cos2α·sinα
=2sinα·cosα^2+(cosα^2-sinα^2)sinα
=2sinα(1-sinα^2)+(1-sinα^2-sinα^2)sinα
=2sinα-2sinα^3+sinα-2sinα^3
=3sinα-4sinα^3
sin3a+sin3b=3sina-4sina^3+3sinb-4sinb^2
=3(sina+sinb)-4(sina^3+sinb^3)
=3(sina+sinb)-4(sina+sinb)(sina^2-sinasinb+sinb^2)
上面都有值呢,你自己带进去算

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