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[tex]\sum_{k=0}^{n}x^{k}=\frac{1-x^{n+1}}{1-x}[/tex]

Hint: Multiply both sides by 1-x.

b) Substitute x = e^t in the formula for the partial geometric series.

c) Perform a series expansion of both sides to second order in t.

- #3

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Shouldn't that be (1+i)^3 - i^3?

If you expand (i+1)^3, you see that i^3 cancels and then you get a combinaton of the summaton of i^2 and i and of 1.

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