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Show that \(\frac 1x \ge 3 - 2\sqrt{x}\) for all positive real numbers x. Describe when we have equality.

 

Hi! Please provide a full solution.

 

Thanks!

 Oct 24, 2018
 #1
avatar+624 
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\(\frac{1}{x} \geq 3 - 2\sqrt{x} \iff \frac{1+2x\sqrt{x} -3x}{x} \geq 0 \iff \frac{(\sqrt{x}-1)^2(\sqrt{x} + \frac{1}{2})}{x} \geq 0\)

 

We can cancel off the \((\sqrt{x}-1)^2\), and the \(x\) on the bottom, since \(x\) must be positive, and squares are always positive.

 

This leaves us with \(\sqrt{x} + \frac{1}{2} \geq 0\) which is true for all positive \(x\)

 

Hence this inequality also applies for all positive \(x\).

 

Equality will apply when \(x = 1\), since \(\sqrt{x}+\frac{1}{2}\) is strictly positive, and \(\frac{1}{x}\) is strictly positive for \(x \neq 0\).

 

Source: 

1) https://math.stackexchange.com/questions/1929829/show-that-frac-1x-ge-3-2-sqrtx-for-all-positive-real-numbers-x-descri

2) https://math.stackexchange.com/questions/1929829/show-that-frac-1x-ge-3-2-sqrtx-for-all-positive-real-numbers-x-descri/1929838

 

If it is urgent, this is all I have. If you need something more detailed, I'm sure someone will be able to better assist you soon. 

 

coolcool

 Oct 24, 2018
 #2
avatar+39 
0

Alright, i was hoping for something a little more detailed, but thanks for trying to help!

FencingKat  Oct 25, 2018

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