X is Real

Algebra Level 2

How many real roots exist for the equation ( x + 1 x ) 3 + ( x + 1 x ) = 0 ? \large (x+\dfrac{1}{x})^3+(x+\dfrac{1}{x})=0 \ ?


The answer is 0.

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10 solutions

Simple: You can't put x=0. now, If you put 'x' as a negative number, the left side of the equation becomes negative and If you put 'x' as a positive number, the left side of the equation becomes positive. So there is no solution of the equation in the real line.

Intelligent observation. Cool solution really.

Sanjeet Raria - 6 years, 7 months ago

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I just thought that we can't substitute any number to the given expression so I answered 0.. :D

Mark Vincent Mamigo - 6 years, 6 months ago

Nice thought

Jordi Bosch - 6 years, 7 months ago
Sanjeet Raria
Nov 6, 2014

Putting a = ( x + 1 x ) a=(x+\frac{1}{x}) We get, a 3 = a a^3=-a a 2 = 1 , s i n c e a 0 \Rightarrow a^2=-1, \space since \space a \ne 0 for real x x .

Now this equation clearly can't be satisfied with any real number. Hence there are no real solutions

Hey I thought the same

Lokesh Agrawal - 6 years, 7 months ago

Its like you were in my head

Ceesay Muhammed - 6 years, 6 months ago
Achraf Laamoum
Nov 8, 2014

W i t h x 0 With\quad x\quad \neq \quad 0 F o r x > 0 w e h a v e : ( x + 1 x ) 3 + ( x + 1 x ) > 0 For\quad x\quad >\quad 0\quad we\quad have\quad :\quad { (x+\frac { 1 }{ x } ) }^{ 3 }+\quad (x+\frac { 1 }{ x } )\quad >\quad 0 F o r x < 0 w e h a v e : ( x + 1 x ) 3 + ( x + 1 x ) < 0 For\quad x\quad <\quad 0\quad we\quad have\quad :\quad { (x+\frac { 1 }{ x } ) }^{ 3 }+\quad (x+\frac { 1 }{ x } )\quad <\quad 0

So there is no reel roots for the equation

Curtis Clement
Mar 22, 2015

Substitute y = x + 1 x \ y = x + \frac{1}{x} such that : y 3 + y = y ( y 2 + 1 ) = 0 y^3 +y = y(y^2+1) = 0 Clearly only y=0 has any chance of producing a real solution. However, it is clear to see that x + 1 x = x 2 + 1 x = 0 x + \frac{1}{x} = \frac{x^2+1}{x} = 0 which doesn't produce a real solution. Hence, there are no real solutions.

Sean Roberson
Nov 11, 2014

Here's an easy way to kill it. Make the substitution u = x + 1 x u = x+\frac{1}{x} . Then we have u 3 = u u^3=-u , or u 2 = 1 u^2 =-1 , which clearly admits no real solutions.

(x+(1/x))((x+(1/x))^2 +1)=0

By Zero Product Property, x + (1/x) = 0 and/or (x + (1/x))^2 = -1. Latter is not possible for real x.

Also x^2 + 1 = 0 is not possible for real x. Hence no real roots exist for this equation.

( x + 1 x ) 3 + ( x + 1 x ) = 0 (x+\frac{1}{x})^{3}+(x+\frac{1}{x})=0 . As x + 1 x 0 x+\frac{1}{x} \ne 0 , we have ( x + 1 x ) 2 = 1 (x+\frac{1}{x})^{2}=-1 but an squared real number is never going to be negative.

Abdul Basith S
Aug 18, 2015

If x+1/x=0 roots are imaginary Then, x^2+1/x^+3=0 Put t=x^2 and solve Roots are imaginary

Rohit Yadav
Dec 2, 2014

(x+1/x)^3 +(x + 1/x)=0 , (x+1/x)((x +1/x)^2 + 1)=0 , (x^2 + 1)( (x + 1/x)^2 + 1) =0 , here in both bracket value can not be zero ,because these are positive .Due to this right hand side value can note be zero, this is grater than zero. therefore .there is no any real solution for given equation,

I did the same way, but putting it with LaTex to make it easy. ( x + 1 / x ) 3 + ( x + 1 / x ) = ( x + 1 / x ) ( ( x + 1 / x ) 2 + 1 ) = 0. S i n c e x 0 , ( x 2 + 1 ) ( ( x + 1 / x ) 2 + 1 ) = 0 , here in both bracket value can not be zero these are positive .Due to this, right hand side value can note be zero this is grater than zero. there is no any real solution for given equation. (x+1/x)^3 +(x + 1/x)=(x+1/x)((x +1/x)^2 + 1)=0. ~~~~~~Since~~x\ne 0,\\ (x^2 + 1)( (x + 1/x)^2 + 1) =0 ,\text{ here in both bracket value can not be zero} \\ \because \text{these are positive .Due to this, right hand side value can note be zero}\\\text{ this is grater than zero.}\\ \therefore \text{there is no any real solution for given equation.}

Niranjan Khanderia - 6 years, 3 months ago
Anchit Virmani
Nov 22, 2014

I solved it this way : Let f(x) = (x+ 1/x)^3 + (1 + 1/x) Expand and take LCM. Now find the number of sign changes of co-effecients , that tells the number of + roots , which is 0 here. Now find f(-x) , again repeat the same procedure. That is the number of -ve roots. which is 0 here . So all roots are complex , there are 6 complex roots.

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