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sinα-sinβ=-1/3(1)cosα-cosβ=1/2(2)兩式相乘:sinαcosα-sinαcosβ-cosαsinβ+sinβcosβ=-1/6 1/2sin2α-sin(α+β)+1/2sin2β=-1/6 sin(α+β)=1/2sin2α+1/2sin2β+1/6 =1…
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