#oxford maths entrance exam question

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frozen prairie
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I have this question here which I tried to do but I could not work out how to do part iii. After a while I tried look at the mark scheme as well but I still couldn't quite understand it. Why is it significant that k>i? Why must that be true for ai to be constant

ashen inletBOT
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modest acornBOT
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Element118

frozen prairie
hearty wasp
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Essentially what you want to prove is that the coefficient of $x^i$ would eventually be as in $p_i(x)$

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Element118

hearty wasp
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$p_{i+1}(x)=(1+x^{i+1})p_i(x)=p_i(x)+x^{i+1}p_i(x)$

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Element118

hearty wasp
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and only the $p_i(x)$ part matters here

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Element118

frozen prairie
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