解：令t=a-π/3,则2a-π/6=2t
所以cost=3/5,sint=√(1-(cost)^2)=4/5
sin(2a-π/6)=sin2t=2sint*cost=2*3/5*4/5=24/25
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
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