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[15分]Trigo equation問題一條...

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請詳列步驟 ABC is a triangle If (cosA)^2 - (cosB)^2 - (cosC)^2 = -1, show that ABC is a right-angled triangle [之前prove左(cosA)^2 - (cosB)^2 + (sinC)^2 = 2cosAsinBsinC] 更新: 嘩..高明高明...佩服佩服...過左4個鐘我一定選你做最佳解答... 你是大學生對吧-v-？

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• (20點) F.2 Maths Question (Pythagors' Theorem)

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(cosA)^2 - (cosB)^2 - (cosC)^2 = -1 (cosA)^2 - (cosB)^2 - (cosC)^2 = -(cosC)^2 - (sinC)^2 (cosA)^2 - (cosB)^2 + (sinC)^2 = 0 2cosAsinBsinC=0 i.e. cosA or sinB or sinC = 0 i.e. A = 90 or B or C = 0 so ABC is a right-angled triangle

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