by Kurt » Wed Apr 11, 2007 11:58 pm
You might be able to use the identity [unparseable or potentially dangerous latex formula][unparseable or potentially dangerous latex formula], but I wouldn't know how to solve for the complex roots of that easily at all....although it does look like it has nice symmetry.
Instead, it's easy to see that [unparseable or potentially dangerous latex formula] is just a geometric sequence, so the sum is:
[unparseable or potentially dangerous latex formula].
And so [unparseable or potentially dangerous latex formula], or:
[unparseable or potentially dangerous latex formula]. Simplifying, we get
[unparseable or potentially dangerous latex formula].
This fortunately factors:
[unparseable or potentially dangerous latex formula].
Thus solutions to [unparseable or potentially dangerous latex formula] are what we are interested in.
[unparseable or potentially dangerous latex formula], so the only solution common to both factors is [unparseable or potentially dangerous latex formula]. However, this solution is excluded because of the denominator in the actual polynomial. Therefore, all of the solutions are distinct.
So all solutions are expansions of [unparseable or potentially dangerous latex formula].
By De Moivre's Theorem, [unparseable or potentially dangerous latex formula]. (And the exact same thing, with 17s, for the other root's case.)
In this problem, all [unparseable or potentially dangerous latex formula] or [unparseable or potentially dangerous latex formula], so the sum of the five lowest [unparseable or potentially dangerous latex formula] is simply the sum of the five lowest fractions with positive integer numerators with 17 or 19 in the denominator.
Since [unparseable or potentially dangerous latex formula] (the five lowest [unparseable or potentially dangerous latex formula]):
[unparseable or potentially dangerous latex formula].
So [unparseable or potentially dangerous latex formula].