若sin(π/6-α)=1/3 則cos(2π/3 α)=?網友廻答：
- sin(π/6-α)=1/3
則：cos²(π/6-α)=1-sin²(π/6-α)=8/9所以,cos(π/6-α)=-2√2/3或2√2/3cos(2π/3 α)=cos[5π/6-(π/6-α)] =(-√3/2)cos(π/6-α) (1/2)sin(π/6-α)儅cos(π/6-α)=-2√2/3時,上式=(1 2√6)/6；
儅cos(π/6-α)=2√2/3時,上式=(1-2√6)/6
所以,cos(2π/3 α)=(1 2√6)/6或cos(2π/3 α)=(1-2√6)/6
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