赞 物是人非who怕who 幼苗
共回答了16个问题采纳率：87.5% 举报
sin2A+sin2B+sin2C
= 2sin(A+B)cos(A-B)+sin2C
= 2sinCcos(A-B)+2sinCcosC
= 2sinC*[cos(A-B)+cosC]
= 2sinC*[cosAcosB+sinAsinB-cos(A+B)]
= 2sinC*[cosAcosB+sinAsinB-cosAcosB+2sinAsinB]
= 4sinC*sinA*sinB
= 2sin(A+B)cos(A-B)+sin2C
= 2sinCcos(A-B)+2sinCcosC
= 2sinC*[cos(A-B)+cosC]
= 2sinC*[cosAcosB+sinAsinB-cos(A+B)]
= 2sinC*[cosAcosB+sinAsinB-cosAcosB+2sinAsinB]
= 4sinC*sinA*sinB
1年前
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