已知a=y+20,b=y+19,c=y+21,那么代数式a平方+b平方+c平方-ab-bc-ca的值?

a平方+b平方+c平方-ab-bc-ca=a(a-b)+b(b-c)+c(c-a)=(y+20)(y+20-y-19)+(y+19)(y+19-y-21)+(y+21)(y+21-y-20)=y+20-2y-38+y+21=3