Man This drives Me crazy!!!!!!

Algebra Level 4

Given that x 2 + x + 1 = 0 x^2+x+1=0 , find the value of ( x + 1 x ) 2 + ( x 2 + 1 x 2 ) 2 + + ( x 6 + 1 x 6 ) 2 (x+\dfrac{1}{x})^2+(x^2+\dfrac{1}{x^2})^2+\dots +(x^6+\dfrac{1}{x^6})^2 .


The answer is 12.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

6 solutions

First, given that x 2 + x + 1 = 0 x^{2} + x + 1 = 0 , we have that

x + 1 x = x 2 + 1 x = x x = 1 x + \frac{1}{x}\ = \frac{x^{2} + 1}{x} = \frac{-x}{x} = -1 .

Next, since ( x + 1 x ) 2 = x 2 + 1 x 2 + 2 (x + \frac{1}{x})^{2} = x^{2} + \frac{1}{x^{2}} + 2 ,

we see that x 2 + 1 x 2 = ( 1 ) 2 2 = 1 x^{2} + \frac{1}{x^{2}} = (-1)^{2} - 2 = -1 .

Now ( x + 1 x ) ( x 2 + 1 x 2 ) = x 3 + 1 x 3 + ( x + 1 x ) (x + \frac{1}{x})(x^{2} + \frac{1}{x^{2}}) = x^{3} + \frac{1}{x^{3}} + (x + \frac{1}{x}) ,

and so x 3 + 1 x 3 = ( 1 ) ( 1 ) ( 1 ) = 2 x^{3} + \frac{1}{x^{3}} = (-1)(-1) - (-1) = 2 .

Next, ( x 2 + 1 x 2 ) 2 = x 4 + 1 x 4 + 2 (x^2 + \frac{1}{x^{2}})^{2} = x^{4} + \frac{1}{x^{4}} + 2 ,

and so x 4 + 1 x 4 = ( 1 ) 2 2 = 1 x^{4} + \frac{1}{x^{4}} = (-1)^{2} - 2 = -1 .

Continuing, we have ( x 2 + 1 x 2 ) ( x 3 + 1 x 3 ) = x 5 + 1 x 5 + ( x + 1 x ) (x^{2} + \frac{1}{x^{2}})(x^{3} + \frac{1}{x^{3}}) = x^{5} + \frac{1}{x^{5}} + (x + \frac{1}{x}) ,

and so x 5 + 1 x 5 = ( 1 ) ( 2 ) ( 1 ) = 1 x^{5} + \frac{1}{x^{5}} = (-1)(2) - (-1) = -1 .

Now ( x 3 + 1 x 3 ) 2 = x 6 + 1 x 6 + 2 (x^{3} + \frac{1}{x^{3}})^{2} = x^{6} + \frac{1}{x^{6}} + 2 ,

and so x 6 + 1 x 6 = 2 2 2 = 2 x^{6} + \frac{1}{x^{6}} = 2^{2} - 2 = 2 .

Finally, evaluating the given expression, we have an answer of

( 1 ) 2 + ( 1 ) 2 + 2 2 + ( 1 ) 2 + ( 1 ) 2 + 2 2 = 12 (-1)^{2} + (-1)^{2} + 2^2 + (-1)^{2} + (-1)^{2} + 2^{2} = \boxed{12} .

The reason why this is so nice, is because x x is a cube root of unity, hence we get the above pattern.

Calvin Lin Staff - 6 years, 9 months ago

Log in to reply

Hmmm... Yes, of course, as ( x 1 ) ( x 2 + x + 1 ) = x 3 1 (x - 1)(x^{2} + x + 1) = x^{3} - 1 . So the roots of x 2 + x + 1 = 0 x^{2} + x + 1 = 0 are complex but satisfy x 3 = 1 x^{3} = 1 , so

x 1 + 3 n = x , x 2 + 3 n = x 2 x^{1 + 3n} = x, x^{2 + 3n} = x^{2} and x 3 n = x 3 = 1 x^{3n} = x^{3} = 1

for all integers n n . Thus

x + 1 x = x + x 2 = 1 x + \frac{1}{x} = x + x^{2} = -1 and

x 2 + 1 x 2 = x 2 + x = 1 x^{2} + \frac{1}{x^{2}} = x^{2} + x = -1 and

x 3 + 1 x 3 = 1 + 1 = 2 x^{3} + \frac{1}{x^{3}} = 1 + 1 = 2 .

The cycle then continues with period 3 3 . So I didn't need to bash away after all. Oh, well. I'll remember next time, hopefully.

Brian Charlesworth - 6 years, 9 months ago

Same approach as mine

Parth Lohomi - 6 years, 9 months ago

too much calculs, it's boring ..

Mirah Hamza - 6 years, 8 months ago
Sandeep Bhardwaj
Sep 16, 2014

From the given x 2 + x + 1 = 0 x^{2} +x+1=0 we know x = w x=w OR x = w 2 x=w^{2}

( where w being cube root of unity i.e. 1 + 3 i 2 \frac{-1+\sqrt{3}i}{2} )

(and we know that 1 w = w 2 \frac{1}{w}=w^{2} and 1 w 2 = w \frac{1}{w^{2}}=w because w 3 = 1 w^{3}=1

and 1 + w + w 2 = 0 1+w+w^{2}=0 => w + w 2 = 1 w+w^{2}=-1

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

x 2 + 1 x 2 = w 2 + 1 w 2 x^{2}+\frac{1}{x^{2}}=w^{2}+\frac{1}{w^{2}} =-1

x 3 + 1 x 3 = 1 + 1 x^{3}+\frac{1}{x^{3}}=1 + 1 =2

[ x n = 1 x^{n}=1 whenever n is a multiple of 3 ]

So value comes = 1 + 1 +4 +1 +1 +4 = 12 \boxed{12}

Sai Ram
Aug 18, 2015

It is given that x 2 + x + 1. x^2+x+1.

Now ,

x 2 + x + 1 = 0 x 2 + 1 = x x ( x + 1 x ) = x x + 1 x = 1 . x^2+x+1 =0 \Rightarrow x^2+1=-x \Rightarrow x(x+\dfrac{1}{x}) = -x \Rightarrow \boxed {x+\dfrac{1}{x}=-1}.

Squaring on both sides,

x 2 + 1 x 2 + 2 = 1 x 2 + 1 x 2 = 1 . x^2+\dfrac{1}{x^2}+2=1 \Rightarrow \boxed{ x^2+\dfrac{1}{x^2}=-1}.

Now cubing the first equation, we get , x 3 + 1 x 3 + 3 ( x + 1 x ) = 1 x 3 + 1 x 3 = 2 . x^3+\dfrac{1}{x^3}+3(x+\dfrac{1}{x}) = -1 \Rightarrow \boxed{x^3+\dfrac{1}{x^3}=2}.

Next , squaring the second equation , we get ,

x 4 + 1 x 4 + 2 = 1 x 4 + 1 x 4 = 1 . x^4+\dfrac{1}{x^4}+2 = 1 \Rightarrow \boxed{x^4+\dfrac{1}{x^4}=-1}.

Now , by multiplying second and third equations ,

x 5 + 1 x 5 + x + 1 x = 2 x 5 + 1 x 5 = 1 . x^5+\dfrac{1}{x^5}+x+\dfrac{1}{x}=-2 \Rightarrow \boxed{x^5+\dfrac{1}{x^5}=-1}.

Now , by multiplying first and fifth equations ,

x 6 + 1 x 6 + x 4 + 1 x 4 = 1 x 6 + 1 x 6 = 2 . x^6+\dfrac{1}{x^6}+x^4+\dfrac{1}{x^4}=1 \Rightarrow \boxed{x^6+\dfrac{1}{x^6}=2}.

Now, the sum of their squares is

( 1 ) 2 + ( 1 ) 2 + ( 2 ) 2 + ( 1 ) 2 + ( 1 ) 2 + ( 2 ) 2 = 1 + 1 + 4 + 1 + 1 + 4 = 12 . (-1)^2+(-1)^2+(2)^2+(-1)^2+(-1)^2+(2)^2 = 1+1+4+1+1+4 = \boxed{12}.

Dylan Cope
Sep 16, 2014

Solving x 2 + x + 1 {x}^{2}+{x}+1 using the quadratic formula or what not to find: x = 1 2 ± 3 2 i x=\frac{1}{2}\pm \frac{\sqrt{3}}{2}i , this complex number can then be written in trigonometric form: x = cos 2 π 3 ± sin 2 π 3 x=\cos\frac{2\pi}{3}\pm\sin\frac{2\pi}{3} . De Moirve's theorem then says ( cos θ + i sin θ ) n = cos n θ + sin n θ {(\cos\theta+i\sin\theta)}^{n}=\cos n\theta+\sin n\theta , hence it can be figured that z n + 1 z n = 2 cos n θ {z}^{n}+\frac{1}{{z}^{n}}=2\cos n\theta . Where z z is a complex number. Since any value of x x is complex, all the terms in the expression we are trying to find can be simplified to k = 1 6 4 cos 2 2 k π 3 \sum _{ k=1 }^{ 6 }{ 4\cos ^{2}{ \frac { 2k\pi }{ 3 } } } , which can then quite easily evaluated to 12. I found that the most interesting way to solve it :)

Prasoon Verma
Dec 10, 2014

taking x^2 as common in the given equation we get (x+1/x)=-1, squaring this we get the first term and by opening square we get our second term squaring this again got our second term and applying binomial got our more terms the will be 1+1+4+1+1+4

Ronald Salim
Sep 16, 2014

x^2 + x +1=0 multiply x

x^3 + x^2 + x = 0

x^3 + x^2 + x + 1 = 1

x^3 + 0 = 1

x^3 = 1

x^3 + 1/x^3 = 2, by square

x^6 + 1/x^6 = 2

since x not equal to 0, x^2 + x +1=0 divide x

x + 1/x = -1, by square

x^2 + 1/x^2 = -1, by square

x^4 + 1/x ^4 = -1

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

x^5 + 1/x^5= -1

sum of all square is 12

Ronald, bagus pendekatannya. Sedikit komentar: saya kira, sesudah didapatkan bahwa x^3 = 1, maka semua x dengan pangkat yang lebih tinggi bisa direduksi. Misalnya x^4 jadi sama dengan x^1, kemudian x^5 jadi sama dengan x^2, dst.nya.

Phil Santosa - 6 years, 8 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...