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Algebra Level 2

If x 2 + x + 1 = 0 x^2+x+1= 0 , what is the value of x 6 + 1 x 6 x^6 + \frac 1 {x^6} ?

Credits to Engr. Maling


The answer is 2.

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5 solutions

Rick B
Jan 14, 2015

x x is a cube root of unity:

x 2 + x + 1 = x 3 1 x 1 = 0 x 3 1 = 0 x 3 = 1 x^2+x+1 = \dfrac{x^3-1}{x-1} = 0 \implies x^3-1 = 0 \implies x^3 = 1

So x 6 + 1 x 6 = ( x 3 ) 2 + 1 ( x 3 ) 2 = 1 2 + 1 1 2 = 1 + 1 = 2 x^6+\dfrac{1}{x^6} = (x^3)^2+\dfrac{1}{(x^3)^2} = 1^2+\dfrac{1}{1^2} = 1+1 = \boxed{2}

Curtis Clement
Jan 15, 2015

Using the quadratic formula (with a=b=c=1) gives x 1 x_{1} = - 1 2 \frac{1}{2} + 3 2 \frac{\sqrt{3}}{2} j {j} and x 2 x_{2} = - 1 2 \frac{1}{2} - 3 2 \frac{\sqrt{3}}{2} j {j} , where j {j} = 1 \sqrt{-1} .

Using De Moivre's: x 1 6 x^6_1 = [- 1 2 \frac{1}{2} + 3 2 \frac{\sqrt{3}}{2} j {j} ]^6 = [Cos 2 π 3 \frac{2\pi}{3} + j {j} Sin 2 π 3 \frac{2\pi}{3} ]^6 = Cos4 π \pi + j {j} Sin 4 π \pi = 1.

Similarily: x 2 6 x^6_2 = [- 1 2 \frac{1}{2} - 3 2 \frac{\sqrt{3}}{2} j {j} ]^6 = [Cos 4 π 3 \frac{4\pi}{3} + j {j} Sin 4 π 3 \frac{4\pi}{3} ]^6 = Cos8 π \pi + j {j} Sin 8 π \pi = 1.

\therefore x 6 x^{6} + 1 x 6 \frac{1}{x^6} = 2

Beautiful solution. Nice use of De Moivre's !

Samanvay Vajpayee - 6 years, 2 months ago
John Aries Sarza
Jan 30, 2015

Since x 2 + 1 x 2 = 1 { x }^{ 2 }+\frac { 1 }{ { x }^{ 2 } } =-1 .Cube it ( x 2 + 1 x 2 = 1 ) 3 { { \left( { x }^{ 2 }+\frac { 1 }{ { x }^{ 2 } } =-1 \right) } }^{ 3 } Produces x 6 + 3 ( x 2 + 1 x 2 ) + 1 x 6 = 1 { x }^{ 6 }+3\left( { x }^{ 2 }+\frac { 1 }{ { x }^{ 2 } } \right) +\frac { 1 }{ { x }^{ 6 } } =-1 Substitute x 6 + 3 ( 1 ) + 1 x 6 = 1 { x }^{ 6 }+3\left( -1 \right) +\frac { 1 }{ { x }^{ 6 } } =-1 x 6 + 1 x 6 = 2 { x }^{ 6 }+\frac { 1 }{ { x }^{ 6 } } =\boxed{2}

Vaibhav Kandwal
Jan 10, 2015

Solution to the first equation is ( 1 ) 2 / 3 (-1)^{2/3}

Simply, putting the value in second equation, we get 1 + 1 = 2 \boxed{1+1=2}

Nikhil Jaiswal
Jan 9, 2015

solution of equation 1 are omega and omega square put the value in the question asked so we get 1+1=2 answer

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