◆ わからない問題はここに書いてね 180◆
>>376,531 100ﾟ
∠BCD = θ とおく。
∠ADB = ∠CBD = 30°(錯角), ∠BAD = π-∠ABC = 140°, ∠BDC=π-∠CBD-∠BCD =150°-θ,
sinθ/sin(150°-θ) = sin(∠BCD)/sin(∠BDC) = BD/BC = BD/AB = sin(∠BAD)/sin(∠ADB) = sin(140ﾟ)/sin(30ﾟ) = 2sin(40ﾟ) = sin(80ﾟ)/cos(40ﾟ) = sin(100ﾟ)/sin(50ﾟ)
θ =100ﾟ
∠BCD = θ とおく。
∠ADB = ∠CBD = 30°(錯角), ∠BAD = π-∠ABC = 140°, ∠BDC=π-∠CBD-∠BCD =150°-θ,
sinθ/sin(150°-θ) = sin(∠BCD)/sin(∠BDC) = BD/BC = BD/AB = sin(∠BAD)/sin(∠ADB) = sin(140ﾟ)/sin(30ﾟ) = 2sin(40ﾟ) = sin(80ﾟ)/cos(40ﾟ) = sin(100ﾟ)/sin(50ﾟ)
θ =100ﾟ
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