sin(x+y)
附圖,∠BEO
=
90∘,∠BAO =
90∘,∠ACB =
90∘,∠ADE =
90∘。
如果∠AOD =
x∘
,∠BOA = y∘,則 ∠ABC
= x∘, 直角三角形BOE,sin(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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