Negative Exponents

Algebra Level 1

If x 2 + x 2 = 3 x^2+x^{-2}=3 , find x x 1 |x-x^{-1}| .


The answer is 1.

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1 solution

Justin Wong
Apr 13, 2014

The goal is to get the exponents of 2 and -2 into 1 and -1. A feasible way is to use identities relating to sums and differences of squares. Since the answer asks for x 1 x |x-\frac{1}{x}| , it could be nice to use ( a b ) 2 = a 2 2 a b + b 2 (a-b)^2=a^2-2ab+b^2 . Plugging in x x for a a and 1 x \frac{1}{x} for b b , we get ( x 1 x ) 2 = x 2 2 + 1 x 2 (x-\frac{1}{x})^2=x^2-2+\frac{1}{x^2} . This works out nicely because we have two of the terms stated in the original equation and a constant. Now we can rewrite that equation substituting in our previous work: ( x 1 x ) 2 + 2 = 3 (x-\frac{1}{x})^2+2=3 , so ( x 1 x ) 2 = 1 (x-\frac{1}{x})^2=1 . Taking the square root gives x 1 x = ± 1 x-\frac{1}{x}=\pm 1 , so x x 1 = 1 |x-x^{-1}|=\boxed{1} .

Wait but x = 3 ± 5 2 x = \sqrt{\frac{3 \pm \sqrt{5}}{2}} , and with those values for x x , x + x 1 = 2.236... |x + x^{-1} | = 2.236... .

I got this by doing the following:

Subsitution - Let y = x 2 y = x^2 .

The equation then becomes:

y + 1 y = 3 y + \frac{1}{y} = 3

y 2 3 y + 1 = 0 \rightarrow y^2 - 3y + 1 = 0

y = 3 ± 5 2 \rightarrow y = \frac{3 \pm \sqrt{5}}{2}

Replacing the substitution made at the beginning:

x = y = 3 ± 5 2 x = \sqrt{y} = \sqrt{\frac{3 \pm \sqrt{5}}{2}}

Milly Choochoo - 7 years, 1 month ago

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Hm, I'm trying to see what could account for the differences...Wolfram Alpha is not on my side it seems, but I don't see the error in mine. @Calvin Lin could you help? Also, the last line should be plus or minus, Milly.

There were 47 "solvers" out of 48 attempters... that would mean a lot of people got it wrong...

EDIT: Wolfram now likes me? lol here is what i got for the 4 solutions and here is me plugging in a value for x. @Milly Choochoo I see that in your first line you did x + x 1 x+x^{-1} , but the + should be a -.

Justin Wong - 7 years, 1 month ago

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Oh I guess my comment didn't post properly. After that comment, I posted the following:

GAAAAAAAAAAAAAAAAAAAARGGGGHHH IT WAS A - NOT A + !

CURSE YOU BAD READING SKILLS!

Milly Choochoo - 7 years, 1 month ago

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@Milly Choochoo Lol it's fine, it taught me to check my problems' solutions before posting :)

Justin Wong - 7 years, 1 month ago

the easiest way is=>

make x^2+x^-2 -2=1; i.e.(x-1/x)^2=1 i.e. x-1/x=1;

Suchit Kumar - 7 years, 1 month ago

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