a的平方+b的平方+c的平方-ab-bc-ca
=(2a²+2b²+2c²-2ab-2bc-2ac)÷2
=[(a-b)²+(b-c)²+(a-c)²]÷2
=[(1/5+20-1/5-19)²+(1/5+19-1/5-21)²+(1/5+20-1/5-21)²]÷2
=[1²+(-2)²+(-1)²]÷2
=6÷2
=3