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Algebra Level 3

If x + 1 x = 1 x + \frac{1}{x} = 1 , then find the value of

x + x 4 + x 7 + x 10 + x 19 + x 22 + x 25 + x 31 + x 34 + x 40 . x+{ x }^{ 4 }+{ x }^{ 7 }+{ x }^{ 10 }+{ x }^{ 19 }+{ x }^{ 22 }+{ x }^{ 25 }+{ x }^{ 31 }+{ x }^{ 34 }+{ x }^{ 40 }.


The answer is 0.

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3 solutions

if x + 1 x = 1 x+\frac{1} {x} =1 then x 2 x + 1 = 0 x^2-x+1=0

( x + 1 ) ( x 2 x + 1 ) (x+1)(x^2-x+1) is a factor of x ( 1 + x 3 + x 6 + x 9 + x 18 + x 21 + x 24 + x 30 + x 33 + x 39 ) x(1+x^3 +x^6 +x^9 +x^{18} +x^{21}+x^{24} +x^{30} +x^{33} +x^{39} )

( x ) ( x + 1 ) ( x 2 x + 1 ) ( 1 + x 6 + x 18 + x 24 x 27 + 2 x 30 x 33 + x 36 ) (x)(x+1)(x^2-x+1)(1 + x^6 + x^{18} + x^{24} - x^{27} + 2 x^{30} - x^{33} + x^{36})

but x 2 x + 1 = 0 x^2-x+1=0

So ( x ) ( x + 1 ) ( 0 ) ( 1 + x 6 + x 18 + x 24 x 27 + 2 x 30 x 33 + x 36 ) = 0 x)(x+1)(0)(1 + x^6 + x^{18} + x^{24} - x^{27} + 2 x^{30} - x^{33} + x^{36})= \boxed 0

Superb Solution

Hrishik Mukherjee - 6 years, 2 months ago
Aareyan Manzoor
Jan 12, 2015

the roots of the equation x + 1 x = 1 x+ \dfrac{1}{x}=1 are ω a n d ω 2 \quad -\omega \quad and\quad -\omega^2\quad where ω , ω 2 \quad \omega,\omega^2 \quad are cubic roots of unity note that { ω x = ω , x = 1 m o d 3 ω x = ω 2 , x = 2 m o d 3 ω x = 1 , x = 0 m o d 3 \begin{cases} \omega^{x}= \omega , x=1\mod 3\\ \omega^{x}= \omega^2 , x=2\mod 3\\ \omega^{x}= 1 , x=0\mod 3\\ \end{cases} now , insert the value ω + ( ω ) 4 + ( ω ) 7 + ( ω ) 10 + ( ω ) 19 + ( ω ) 22 + ( ω ) 25 + ( ω ) 31 + ( ω ) 34 + ( ω ) 40 -\omega+{( -\omega )}^{ 4 }+{ (-\omega) }^{ 7 }+{ (-\omega) }^{ 10 }+{( -\omega) }^{ 19 }+{( -\omega) }^{ 22 }+{( -\omega) }^{ 25 }+{( -\omega) }^{ 31 }+{ (-\omega) }^{ 34 }+{ (-\omega) }^{ 40 } from the cases, we get that the expression is ω + ω ω + ω ω + ω ω ω + ω + ω -\omega+\omega-\omega+\omega-\omega+\omega-\omega-\omega+\omega+\omega the positive cancel out the negatives and we are left with 0 \boxed{0}

Sujoy Roy
Dec 27, 2014

x + 1 x = 1 x+\frac{1}{x}=-1

or, x 2 + x + 1 = 0 x^2+x+1=0

or, ( x + 1 ) ( x 2 + x + 1 ) = 0 (x+1)(x^2+x+1)=0 or, x 3 + 1 = 0 x^3+1=0 or, x 3 = 1 x^3=-1 .

Now, 1729 + x + x 4 + x 7 + x 10 + x 19 + x 22 + x 25 + x 31 + x 34 + x 40 1729+x+x^{4}+x^{7}+x^{10}+x^{19}+x^{22}+x^{25}+x^{31}+x^{34}+x^{40}

= 1729 + x x + x x + x x + x + x x x = 1729 =1729+x-x+x-x+x-x+x+x-x-x= \boxed{1729}

Note that ( x + 1 ) ( x 2 + x + 1 ) x 3 + 1 (x+1) ( x^2 + x + 1 ) \neq x^3 + 1 .

I believe what you are thinking of is ( x 1 ) ( x 2 + x + 1 ) (x-1) ( x^2 + x + 1) which gives us x 3 1 x^3 - 1 . However, we may no longer continue with the solution.

What we get is that the value is 1729 + 10 ω 1729 + 10 \omega , where ω \omega is one of the complex cube roots of unity. This is equal to 1724 ± 5 3 i 1724 \pm 5 \sqrt{3} i .

The answer has been updated to 1724.

Calvin Lin Staff - 6 years, 5 months ago

Given the changes to the question, I have updated the answer to 0. Can you update your solution accordingly? Thanks!

Calvin Lin Staff - 6 years, 5 months ago

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