The 9th term is the difference between numbers in a pattern. It can be hard to explain so here goes...
1N goes
1,2,3,4,5,6,7,8,9,10...
2N goes
2,4,6,8,10,12,14,16,18,20...
Each N is a times table 1,2,3,4,5,6 ect. times table.
1,2,3,4,5,6,7,8,9,10...
2N goes
2,4,6,8,10,12,14,16,18,20...
Each N is a times table 1,2,3,4,5,6 ect. times table.
What if it's not a times table e.g. 4,7,10,13,16?
- Work out how much the pattern is going up in. In our case it's 3. This means it's 3N.
- But wait, It isn't the 3 times table. What do you need to add/ take away to get the pattern . In our case it's +1.
- This means the pattern is 3N +1