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數學M2題目trigo

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發問:

given that ABC is a triangle, cos A = sin B sin C Prove that ABC is a right-angled triangle!

最佳解答:

giventhat ABC is a triangle,CosA=SinBSinC,Prove that ABC is a right-angled triangle! Sol CosA=Cos(π-B-C)=-Cos(B+C)=SinBSinC-CosBCosC SinBSinC-CosBCosC=SinBSinC CosBCosC=0 CosB=0 orCosC=0 B=π/2 or C=π/2 So ABCis a right-angled triangle!

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