you should of worked out from iii) that:
2I_{1}=\frac{1}{2}, 4I_{3}=\frac{7}{12}
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:
2I_{1}=1-\frac{1}{2}, 4I_{3}=1-\frac{1}{2}+\frac{1}{3}-\frac{1}{4}
then you...