Question

Pattern 1: 3 dots

Pattern 2: 6 dots

Pattern 3: 10 dots

Pattern 4: 15 dots

Pattern 5: 21 dots

I know that pattern 1 to pattern 2 increases by 3 dots, then pattern 2 to pattern 3 increases by 4 dots and so on,etc. How would I go on making an equation about this?