已知三角形ABC的面积等于a²+b²-c²÷4根号3,求∠c

三角形ABC的面积
=1/2abcosc
=1/2absinc
=（a^2+b^2-c^2)/4根号3
=2abcosc/4根号3
[因为cosc= (a^2+b^2-c^2)/2ab]
所以 1/2absinc=2abcosc/4根号3
根号3 *sinc=cosc,tanc=1/根号3
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