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The hint is using power series. So I use Taylor's formula but still cannot reduce it. Any other hints?

pjpj
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1 Answers1

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Notice that $$\frac1{1+x+x^2}=\frac{1-x}{1-x^3}=(1-x)(1+x^3+x^6+\cdots).$$

Angina Seng
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