Just a century

Algebra Level 5

a = 0 100 b = 1 100 x b a \large \displaystyle \sum^{100}_{a=0} \sum^{100}_{b=1}x_{b}^{a}

Let x 1 , x 2 , x 3 , , x 100 x_{1},x_{2},x_{3},\ldots,x_{100} be the roots of the equation x 100 + x + 1 = 0 x^{100}+x+1=0 . Evaluate the summation above.


The answer is -99.

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1 solution

Aareyan Manzoor
Dec 1, 2015

first off, one rule all should remember is that if s 1 = s 2 = . . . = s n = 0 s_1=s_2=...=s_n=0 , then p 1 = p 2 = . . . = p n = 0 p_1=p_2=...=p_n=0 . this is true since in newtons sum p n p_n only contains terms upto s n s_n .proved here . so we see by vietas that s 1 = s 2 = . . . = s 98 = 0 s_1=s_2=...=s_{98}=0 we see that the sum translates to ( x 0 ) + ( x 99 ) + ( x 100 ) \sum(x^0)+\sum (x^{99})+\sum (x^{100}) now x 100 = x 1 x^{100}=-x-1 ( x 100 ) = ( x ) ( 1 ) = 0 100 = 100 \sum (x^{100})=-\sum(x)-\sum(1)=0-100=-100 x 99 = 1 1 x x^{99}=-1-\dfrac{1}{x} ( x 99 ) = ( 1 ) ( 1 x ) = 100 s 99 s 100 = 100 + 1 = 99 \sum(x^{99})=-\sum(1)-\sum(\dfrac{1}{x})=-100-\dfrac{s_{99}}{s_{100}}=-100+1=-99 so 100 100 99 = 99 100-100-99=\boxed{-99}

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