the key is to write [tex]n^2 + n[/tex] as [tex]n(n+1)[/tex] which can be easily seen as an area of a rectangle with one side length n and the other (n+1). That explains the rectangle made of dots - 3 x 4
Now you only have half of them because of the 1/2. So that explains the lower "triangle" of dots being greyed out - they do not count in the area (upper triangle). So the count of the dark dots corresponds to the original expression.
For n=5 you can draw a similar rectangular matrix with 5 x 6 dots. And count only the upper triangular part.
