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 sin(x+y)
 

 
圖,∠BEO 90,∠BAO 90,∠ACB 90,∠ADE 90

如果∠AOD x ∠BOA y則 ∠ABC x 直角三角形BOEsin(x+y)=¯BE¯OB

sin(x+y)=¯BE¯OB=¯BC+¯CE¯OB=¯BC+¯AD¯OB=¯BC¯OB+¯AD¯OB=¯BC¯OBׯBA¯BA+¯AD¯OBׯOA¯OA=¯BC¯BAׯBA¯OB+¯AD¯OAׯOA¯OB=

cos x sin y+sin x cos y=sin x cos y+cos x sin y

 



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