you should of worked out from iii) that:
now these are the first 2 odd terms of the sequence, if you look at what the question wants you to prove, you can then realise that:
then you can say, therefore for n is odd (n+1)I is blah blah...
and then i reckon you can finish the even off, exact same setting, but with the even members...hope this helps
Edit: Probably best to say then we can deduce
is blah blah...therefore continuing this pattern will lead to:
equals blah blah for n is odd