What's the equation that describes this number sequence:

N | D
3 | 0
4 | 2
5 | 5
6 | 9


I see the pattern, from 3 to 4, it grows by 2, then by 3, 4, etc.

N=the number of sides, and D=the number of diagnals. If you draw a triangle (three sides, so N=3), you have 0 diagnals you can draw from point to point (so D=0), which is why where N=3, D=0. Then, with a square, you have 2 diagnals, pentagon has 5 diagnals, hexagon has 9. Following the patter, N=7 would produce D=14. I drew a heptagon (7 sides), and it did have 14 diagnals. So going by the pattern, where N=8, D=20; N=9, D=27; and N=10, D=35.

Can anyone help me out on this?

~Luke
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