1/2 (a^2 - 2ab + b^2 + b^2 - 2bc +c^2 + c^2 - 2ca + a^2 ) = 0 1/2 { (a-b)^2 + (b-c)^2 +(c-a)^2 } = 0 따라서, (a-b)=0, (b-c)=0, (c-a)=0, 즉 , a=b=c
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