a=1/5+20,b=1/5+19,c=1/5+21所以可得：
a-b=1,b-c=-2,a-c=-1
a^2+b^2+c^2-ab-bc-ca
=1/2(2a^2+2b^2+2c^2-2ab-2bc-2ca)
=1/2[(a-c)^2+(a-b)^2+(b-c)^2]
=1/2(1+1+4)
=3