Quintic Quarrel

Algebra Level 3

Refer to the following equation:

x 5 + x 4 + 1 = 0 x^{5}+x^{4}+1=0

This equation has only one real solution x. Can you find a b s ( x ) abs(x) ?

(Write your answer by two decimal places.)


The answer is 1.32.

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2 solutions

Aryan Sanghi
Jun 4, 2020

y = x 5 + x 4 + 1 y = x^5 + x^4 + 1

y = x 5 + x 4 + x 3 x 3 + 1 y = x^5 + x^4 + x^3 - x^3 + 1

y = x 3 ( x 2 + x + 1 ) ( x 1 ) ( x 2 + x + 1 ) y = x^3(x^2 + x+ 1) - (x - 1)(x^2 + x + 1)

y = ( x 2 + x + 1 ) ( x 3 x + 1 ) y = (x^2 + x+ 1)(x^3 - x + 1)

Now, as ( x 2 + x + 1 ) (x^2 + x + 1) has imaginary roots, x 3 x + 1 x^3 - x + 1 is the one with 1 real root

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

Now, graph of x 3 x x^3 - x is

So, graph of x 3 x + 1 x^3 - x + 1 will be above graph shifted one unit up. It will intersect x-axis at about -1.32. So,

R o o t = 1.32 \boxed{|Root| = 1.32}


Note: I drew graph myself while solving. Above graph from Wolfram Alpha is taken for explaining.

Mahdi Raza
Jun 3, 2020

Is the video similar to my solution?

Aryan Sanghi - 1 year ago

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Yes, kind of..

Mahdi Raza - 1 year ago

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Slight difference. Both Aryan and the BPRP video showed that the answer is the root of the cubic polynomial x 3 x + 1 x^3 - x + 1 . Aryan used graphing/approximation to solve it. BPRP used Cardano's formula to solve it.

Pi Han Goh - 1 year ago

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@Pi Han Goh Ohk sir, thanku.

Aryan Sanghi - 1 year ago

I just wanted to see if anyone else could come up with other solutions.

Deva Craig - 1 year ago

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Did I come with it?

Aryan Sanghi - 1 year ago

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