a-b=5/13
b-c=2/13 两式相加得
a-c=7/13
a^2+b^2+c^2-(ab+bc+ac)
=[(a-b)^2+(b-c)^2+(a-c)^2]/2
=(5/13)^2+(2/13)^2+(7/13)^2
=78/169
=6/13
ab+ac+bc
=a^2+b^2+c^2-6/13
=1-6/13
=7/13