sin(a+π/3)+sina=-4√3/5,-π/2

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sin(a+π/3)+sina=-4√3/5,-π/2
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sin(a+π/3)+sina=-4√3/5,-π/2
sin(a+π/3)+sina=-4√3/5,-π/2

sin(a+π/3)+sina=-4√3/5,-π/2
∵-π/2<a<0,∴π/6-π/2<a+π/6<π/6,∴-π/3<a+π/6<π/6.
∵sin(a+π/3)+sina=-4√3/5,∴2sin(a+π/6)cos(π/6)=-4√3/5,
∴sin(a+π/6)=-4/5,结合-π/3<a+π/6<π/6,得:
cos(a+π/6)=√(1-16/25)=3/5.
∴cosa=cos[(a+π/6)-π/6]=cos(a+π/6)cos(π/6)+sin(a+π/6)sin(π/6)
=(3/5)×(√3/2)+(-4/5)×(1/2)=3√3/10-4/10=(3√3-4)/10.

√3cosa / 2 + 3sina/ 2 = -4√3/5
√3sin(a+β)= -4√3/5 ,tanβ = √3/3
sin(a + π/6) = -4/5
√3sina /2 + cosa /2 = -4/5
100cos²a + 80cosa -11= 0
cosa =