First degree is equal to second degree?

Algebra Level pending

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

If the equation above is true for some real positive value x x , what is the value of x 1024 + 1 x 1024 x^{1024} + \dfrac{1}{x^{1024}} ?


The answer is 2.

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2 solutions

Since x 2 + 1 x 2 = ( x + 1 x ) 2 2 x^{2} + \dfrac{1}{x^{2}} = \left(x + \dfrac{1}{x}\right)^{2} - 2 , if we let x + 1 x = y x + \dfrac{1}{x} = y the given equation becomes

y = y 2 2 y 2 y 2 = ( y 2 ) ( y + 1 ) = 0 y = y^{2} - 2 \Longrightarrow y^{2} - y - 2 = (y - 2)(y + 1) = 0 ,

so either y = 2 y = 2 or y = 1 y = -1 . But as x > 0 x \gt 0 we must have y > 0 y \gt 0 , and so

y = x + 1 x = 2 x 2 + 1 = 2 x x 2 2 x + 1 = ( x 1 ) 2 = 0 x = 1 y = x + \dfrac{1}{x} = 2 \Longrightarrow x^{2} + 1 = 2x \Longrightarrow x^{2} - 2x + 1 = (x - 1)^{2} = 0 \Longrightarrow x = 1 .

Thus x 1024 + 1 x 1024 = 1 + 1 1 = 2 x^{1024} + \dfrac{1}{x^{1024}} = 1 + \dfrac{1}{1} = \boxed{2} .

@Brian Charlesworth Sir, I am thinking about the level.. How about you?

Fidel Simanjuntak - 4 years, 8 months ago

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I'm surprised that, with 54 solvers already, the level hasn't been set yet. It's usually an automatic process, but it can take longer if you don't initially attach a level to the problem when you post it. I would consider this a level 2 problem.

Brian Charlesworth - 4 years, 8 months ago

@Brian Charlesworth No problem.. Thx for your suggestion..

Fidel Simanjuntak - 4 years, 8 months ago

@Brian Charlesworth No problem.. Thx for your suggestion.. By the way, i'm still 14..

Fidel Simanjuntak - 4 years, 8 months ago
Fidel Simanjuntak
Sep 17, 2016

x + 1 x = x ² + 1 x ² x+ \frac{1}{x} = x² + \frac{1}{x²}

Multiply both side by x ²

x ³ + x = x 4 + 1 x³ + x = x^4 + 1

x 4 x ³ x + 1 = 0 x^4- x³ - x +1 =0

If x = 1 x=1 , the equation is complete. Then, we have x = 1 x=1 .

Hence,

x 1024 + 1 x 1024 = 1 + 1 x^{1024} + \frac{1}{x^{1024}} = 1 + 1

= 2 = \boxed{\boxed{2}}

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