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兩邊平方sinα+2sinαsinβ+sin²;β=sin²;θcos²;α+2cosαcosβ+cos²;β=cos²;θ相加sin²;+cos²;=1所以2+2(cosαcosβsinαsinβ)=12+2cos(α-β)=1cos(α-β)=-1/2
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