已知a-b=b-c=3/5,a^2+b^2+c^2=1,则ab+bc+ca得知等于-----

-b=b-c=3/5
a-b=3/5..(1)
b-c=3/5..(2)
(1)+(2)
a-b+b-c=3/5+3/5
a-c=6/5
a-c=3/5+3/5=6/5
[a-b]^2+[a-c]^2+[b-c]^2=2[a^2+b^2+c^2]-2[ab+ac+bc]
[3/5]^2+[6/5]^2+[3/5]^2=2*1-2[ab+ac+bc]
ab+ac+bc=[2-54/25]/2=-2/25