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sinA+sinB=a, cosA+cosB=b, æ±cos(A-B)çå¼,é£ä¹å 为
cos(A-B) = cosAcosB + sinAsinBï¼
(sinA+sinB)^2 + (cosA+cosB)^2 = (sinA)^2 + 2sinAsinB + (sinB)^2 +
(cosA)^2 + 2cosAcosB + (cosB)^2 = a^2 + b^2
äºæ¯ç±(sinx)^2 + (cosx)^2 = 1
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2 + 2(sinAsinB + cosAcosB) = a^2 + b^2
æ¬å·éé¢çå°±æ¯cos(A-B)
æ以cos(A-B) = (a^2 + b^2 - 2) / 2
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