Bi-Quadratic Equation

Level pending

Find the sum of moduli of all roots of:

x^4 - 3x^3 - 2x^2 - 3x +1=0


The answer is 6.

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1 solution

Shikhar Jaiswal
Mar 17, 2014

The given equation can be written as:-

( x 2 4 x + 1 ) ( x 2 + x + 1 ) = 0 (x^2-4x+1)(x^2+x+1)=0

Roots of x 2 4 x + 1 = 0 x^2-4x+1=0 are 2 + 3 2+\sqrt{3} and 2 3 2-\sqrt{3}

Roots of x 2 + x + 1 = 0 x^2+x+1=0 are ω \omega and ω 2 \omega^2 where ω \omega is complex cube root of unity

ω = ω 2 = 1 |\omega|=|\omega^2|=1

\Rightarrow Sum of Moduli of roots are:- ( 2 + 3 ) + ( 2 3 ) + 1 + 1 = 6 (2+\sqrt{3})+(2-\sqrt{3})+1+1=\boxed{6}

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