10-seconds challenge-1

Calculus Level 4

Minimum value of f ( x ) = x + 1 x f(x)=x+\frac{1}{x} for x > 0 x>0 and x 1 x\neq 1 .


This is a part of 10-seconds challenge .

0 2 -1 1 Does Not Exist -2

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3 solutions

Yash Dev Lamba
Mar 4, 2016

Using A M G M AM\geq GM

x + 1 x 2 \frac{x+\frac{1}{x}}{2} \geq x . 1 x \sqrt{x.\frac{1}{x}}

\implies x + 1 x 2 \frac{x+\frac{1}{x}}{2} \geq 1 1

\implies x + 1 x x+\frac{1}{x} \geq 2 2

equality holds when x = 1 x x=\frac{1}{x} \implies x = ± 1 x=\pm 1

But x > 0 x>0 and also x 1 x \neq 1

\implies m i n min ( x + 1 x x+\frac{1}{x} ) \neq 2 2

So the range of given funtion is ( 2 , ) (2,\infty) not [ 2 , ) [2,\infty) .

Then for obvious reasons its minimum value is does not exist .

Its not "not defined" but "does not exist"

Aareyan Manzoor - 5 years, 3 months ago

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I agree with you ..

Yash Dev Lamba - 5 years, 3 months ago

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great. Wait can normal users change their questions choices?

Aareyan Manzoor - 5 years, 3 months ago

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@Aareyan Manzoor No , I asked staff member to change then he changed.

Yash Dev Lamba - 5 years, 3 months ago

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@Yash Dev Lamba yeah thats fine.

Aareyan Manzoor - 5 years, 3 months ago

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@Aareyan Manzoor by the way thnx for suggestion ..

Yash Dev Lamba - 5 years, 3 months ago

@Yash Dev Lamba You said x 1 x \neq 1 but if we take x = 0.9999997 x = 0.9999997 then it's minimum value is near 2

Department 8 - 5 years, 3 months ago

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@Department 8 Taking value more closer to 1 1 gives value near 2 2 but it cannot be 2 2 it can 1.9999999999997 , 2.0000000001 1.9999999999997,2.0000000001 (there are such infinte value near 2 2 ) . In that case we says that minimum does not exists.

Yash Dev Lamba - 5 years, 3 months ago
Akshay Yadav
Mar 4, 2016

Using calculus,

Let, y = x + x 1 y=x+x^{-1}

d y d x = 1 x 2 \frac{dy}{dx}=1-x^{-2}

For minima the derivative must equal to zero,

1 x 2 = 0 1-x^{-2}=0

x = 1 , 1 x=1,-1

But in the question we are given x 1 x≠1 and x > 0 x>0

Hence the minima doesn't exists.

I thought your d y d x \frac{dy}{dx} is wrong , correct d y d x = 1 1 x 2 \frac{dy}{dx}=1-\frac{1}{x^{2}}

Yash Dev Lamba - 5 years, 3 months ago

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Thanks! I was actually thinking about something else while writing the solution.

Akshay Yadav - 5 years, 3 months ago

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Now there is a flaw in solution too

In 6 t h 6th line you write x = 1 x=1 , indeed x = 1 , 1 x=1,-1

then you have to reject x = 1 x=-1 becuase x > 0 x>0

Yash Dev Lamba - 5 years, 3 months ago

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@Yash Dev Lamba This proves my previous point😛. Thanks again.

Akshay Yadav - 5 years, 3 months ago
Kevin Park
Mar 20, 2016

Using the fact that 0.9999... is actually equal to 1, we can say that 0.000...0001 is equal to 0. As minimum value of X is 0.000...0001 and the value is 0, the answer is Does not Exist.

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