What is, what is x1, x1, y1 plus x2, y2 plus all the way to, all the way to xn, yn. Well if we take this whole sum and we divide it by n terms, this is going to be the mean value for x times y. For each of those points, you multiply x times y and you find the mean of all of those products. That's exactly what this is. Well once again, you multiply both sides of this equation by n and you get, you get x1, y1 plus x2, y2 plus all the way, all the way to xn, yn is equal to, is equal to n times the mean of xy's, the mean of the xy's. So n times the mean of the xy's and I think you see where this is going. This term right here, so this term right here is going to be equal to n times the mean of the products of xy.