nth term

nth term = dn + (a - d)

where d is the difference and a is the first term.

If the second difference is constant (and first difference changes),

where a is the first term, d is the difference between the first and second terms and c is the second difference.

Alternatively:

1. If the first difference keeps changing but the second difference is constant then the formula is something to do with 'n²'. Make a table showing the first few terms of 'n²'.
2. In the next column of your table write the differences between the term of 'n²' and your sequence.
3. Find the formula for this new sequence using dn + (a - d)
4. Add it on to n² to give you your final formula

Well-known sequences