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春芽
共回答了14个问题采纳率:100% 举报
(1)、sin^2A+sin^2B-sin^2C
=sin^2A+sin^2B-sin^2(A+B)
=sin^2A+sin^2B-sin^2Acos^2B-2sinAcosBcosAsinB-cos^2Asin^2B
=2sin^2Asin^2B-2sinAcosBcosAsinB
=2sinAsinB(sinAsinB-cosAcosB)
=2sinAsinBcosC=sinAsinB,
——》cosC=1/2,
——》C=60°;
(2)、cosC=(a^2+b^2-c^2)/2ab=1/2,
——》a^2+b^2-9=ab,
——》ab=[(a+b)^2-9]/3=3,
——》S△=1/2*ab*sinC=3√3/4.
1年前
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