Make your own Number Patterns

You can make your own number patterns using coins or matchsticks. Here is an example using coins:

Size=1	Size=2	Size=3	Size=4
1 Coin	6 Coins	15 Coins	28 Coins

How many coins would you need when Size=5 ?

Can you make a formula that will tell you how many coins are needed for any Size? For example Size=20? The formula may look something like