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问题: 化简

解答:

sin(π/5-θ)=lg³√100 = 2/3

原式= cos²[π-(π/5-θ)]-sin[π+(π/5-θ)]
  = cos²(π/5-θ)+sin(π/5-θ)
  = 1-(2/3)²+(2/3)
  = 11/9