Maths Challenge 1

Yes. I forgot to tell everyone: I will put one Maths question and a Chemistry question every week. Suprised? Now, here is the question:

If a = 1990x+1989,b=1990x+1990,c=1990x+1991, find the value of a2+b2+c2-ab-ac-bc?

I hope you will count it and got correct answer. The answer will put on tomorrow.