The triangular numbers
Rule
Each term is: Multiply by the next larger number, and divide by two
Known as: The triangular numbers
Each term is n plus the previous term, as shown in the illustration.
The first 50 terms (starting from n=1) are:
1 3 6 10 15 21 28 36 45 55 66 78 91 105 120 136 153 171 190 210 231 253 276 300 325 351 378 406 435 465 496 528 561 595 630 666 703 741 780 820 861 903 946 990 1035 1081 1128 1176 1225 1275
Example
To get the value for n=13:
- Multiply by the next larger number, giving 182.
- Divide by two, giving 91.
So the result is 91.