a-b=b-c=3/5
所以a-c=(a-b)+(b-c)=6/5
(a-b)^2+(b-c)^2+(a-c)^2=9/25+9/25+36/25=54/25
(a^2-2ab+b^2)+(b^2-2bc+c^2)+(c^2-2ac+a^2)=54/25
2a^2+2b^2+2c^2-2ab-2bc-2ac=54/25
a^2+b^2+c^2-(ab+bc+ac)=27/25
a^2+b^2+c^2=1
所以ab+bc+ac=1-27/25=-2/25