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a-b=3/5① b-c=3/5② 把兩式相加,就可得:a-c=6/5③ 然後將這a-c=6/5平方,再相加,即得: 2(a^2+b^2+c^2)-2(ab+bc+ca)=54/25 因為a^2+b^2+c^2=1,所以ab+bc+ac=-2/25
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