sinA+sinB=sinC,
cosA+cosB=cosC
两个式子分别平方
(sinA+sinB)^2=sinC^2
(cosA+cosB)^2=cosC^2
sinA^2+2sinAsinB+sinB^2=sinC^2
cosA^2+2cosAcosB+cosB^2=cosC^2
两式相加
2+2（cosAcosB+sinAsinB）=1
cosAcosB+sinAsinB=-1/2
cos（A+B)=cosAcosB+sinAsinB=-1/2
救急：已知sinA+sinB=sinC,cosA+cosB=cosC,求cos（A+B)的值
1个回答
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