sin(a-π/3)=√[1-cos^2(a-π/3)]=4/5
sin(2a-π/6)=sin2(a-π/3)=2sin(a-π/3)cos(a-π/3)=2x3/5x4/5=24/25
若cos(a-π/3)=3/5 则sin(2a-π/6)=
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