cos(2π/3+2x)
=-cos[π-(2π/3+2x)]
=-cos(π/3-2x)
=-cos[2(π/6-x)]
=-[1-2sin(π/6-x)^2] (利用倍角关系)
=-[1-2×(5/13)^2]
=-119/169
已知sin(π/6-x)=5/13,求cos(2π/3+2x)的值
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