(a+b+c)^2=A^2+B^2+C^2+2AB+2BC+2AC=0
2AB+2BC+2AC=-(A^2+B^2+C^2)
因为A^2+B^2+C^2≥0
所以-(A^2+B^2+C^2)≤0
2AB+2BC+2AC≤0
ab+bc+ca≤0
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