11 Plus Exams Forum

My son has this for homework and I am stuck, please help. My method of finding the differences does not work.

nth term = (-1) to the power of n+1

so when n is 1,3, 5 you get 1 (ie odd terms are 1)

and when n= 2, 4 you get -1 (even terms are negative)

Is there a formula for this, so far he has had work involving finding the first and second differences giving him either linear or quadratic equations. Your explanation makes perfect sense, I just wondered if there was a method that did not involve any intuition?

No - when signs alternate like this there is usually a power involved - and that's where the difference method breaks down.


ie 2,4,8,16 is the powers of two - if you look at the differences you get 2,4,8 ... which is a bit of a clue.

I would expect that unless they have seen this 'trick' that few chilldren would get the question right so your child might get asked how they worked it out!! That's what I do when I set one like this

Thank you