POLYnomials

Algebra Level 2

The roots of the equation :
x 5 40 x 4 + p x 3 + q x 2 + r x + s = 0 x^5 - 40x^4 + px^3 + qx^2 + rx + s = 0
are in a geometric progression . The sum of their reciprocal is 10. Find the absolute value of s . s.


The answer is 32.

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1 solution

Chris Lewis
Nov 13, 2019

Since the roots are in geometric progression, we can write them as { t u 2 , t u 1 , t , t u , t u 2 } \{ tu^{-2},tu^{-1},t,tu,tu^2 \} . Note that none of the roots can be zero (otherwise the reciprocals wouldn't be defined), so this representation is valid. The reason for this choice is it makes the algebra a little easier:

The sum of the roots is t ( u 2 + u 1 + 1 + u + u 2 ) = 40 t \left(u^{-2}+u^{-1}+1+u+u^2 \right) = 40 (using Vieta).

The sum of the reciprocals of the roots is 1 t ( u 2 + u + 1 + u 1 + u 2 ) = 10 \frac{1}{t} \left(u^{2}+u+1+u^{-1}+u^{-2} \right) = 10 (given in the problem).

From these, we get t 2 = 4 t^2=4 , so that t = ± 2 t=\pm 2 .

The product of the roots (using Vieta again) is just t 5 = s t^5=-s ; from above, this means that s = 2 5 = 32 |s|=2^5=\boxed{32} .

Two mistakes in a single line. It should be " 1 t ( u 2 + u 1 + 1 + u 1 + u 2 ) = 10 \dfrac{1}{t}(u^2+u^1+1+u^{-1}+u^{-2})=10 "

A Former Brilliant Member - 1 year, 7 months ago

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Gah! Thank you - corrected now.

Chris Lewis - 1 year, 7 months ago

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