My Slightly Overcomplicated Problem

Algebra Level 3

Find the minimum root of

x 3 + 1 x 2 1 = x + 6 x 2 \dfrac{x^3+1}{x^2-1} = x+\sqrt[2]{\dfrac{6}{x}}


The answer is 1.5.

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1 solution

Shubhendra Singh
Feb 28, 2015

Given expression can be written as

( x + 1 ) ( x 2 + 1 x ) ( x + 1 ) ( x 1 ) = x 2 + 1 x x 1 = x + 1 x 1 = x + 6 x \dfrac{(x+1)(x^{2}+1-x)}{(x+1)(x-1)}=\dfrac{x^{2}+1-x}{x-1}=x+\dfrac{1}{x-1}=x+\sqrt{\dfrac{6}{x}}

So 1 x 1 = 6 x \dfrac{1}{x-1}=\sqrt{\dfrac{6}{x}}

x 1 = x 6 \Rightarrow x-1=\sqrt{\dfrac{x}{6}}

Whole square and re-arrange to get x 2 13 6 x + 1 = 0 x^{2}-\dfrac{13}{6}x+1=0

= ( x 3 2 ) ( x 2 3 ) \large=(x-\dfrac{3}{2})( x-\dfrac{2}{3})

But x = 2 3 x =\dfrac{2}{3} does not satisfy the equation.

So the answer should be 3 2 \dfrac{3}{2}

But just I was wondering that this doesn't work. try plugging in values. And for some reasons, it doesn't work and it has to be just a root which came with the quadratic, nothing else. The answer has to be 1.5 1.5

Kartik Sharma - 6 years, 3 months ago

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You're right. It's because we squared it, and so an extra root was created.

Plugging in values would have made it work only if we took the negative value of the underroot of 6/x. This problem was indeed caused due to squaring.

Please fix this.

Ayan Jain - 6 years, 3 months ago

I graphed the function given as it was , the graph is given below :

The graph tends to infinity as x tends to 0 , I am telling this since it is not visible from the graph , but you can very well see that x=1 is the asymptote and x= 1.5 is it's least root .

Again proving Kartik's point

@siddharth bhatt , @shubhendra singh

A Former Brilliant Member - 6 years, 3 months ago

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If x = 1, then LHS is not defined..and clearly LHS not equal to RHS ;)

Ayan Jain - 6 years, 3 months ago

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Oops , now I've edited it .

A Former Brilliant Member - 6 years, 3 months ago

Oh ! I just didn't checked the value......sorry guys thanks for correcting me.

Shubhendra Singh - 6 years, 3 months ago

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You are welcome :)

A Former Brilliant Member - 6 years, 3 months ago

Please anyone change the answer to 1.5

siddharth bhatt - 6 years, 3 months ago

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Wow the question is of 200 points now..

Ayan Jain - 6 years, 3 months ago

2/3 is invalid solution, given the form of the equation . (35/27)/(-5/9) = 2/3 +sqrt(9), which is invalid

Mahek Mehta - 6 years, 3 months ago

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