Prove that it is smaller than 6

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August 28th 2011, 06:06 AM
MichaelLight

1 Attachment(s)

Prove that it is smaller than 6

Prove that Attachment 22141.

How do i prove this? thank you...
August 28th 2011, 06:10 AM
Siron

Re: Prove that it is smaller than 6

A possibility is to take the square of both sides.
August 28th 2011, 06:12 AM
alexmahone

Re: Prove that it is smaller than 6

Quote:

Originally Posted by MichaelLight

Prove that Attachment 22141.

How do i prove this? thank you...

$\sqrt{10}+2\sqrt{2}<6$

$\Leftrightarrow 10+8+8\sqrt{5}<36$ (Squaring both sides)

$\Leftrightarrow 8\sqrt{5}<18$

$\Leftrightarrow 320<324$ (Squaring both sides)

which is true.
August 28th 2011, 06:13 AM
CaptainBlack

Re: Prove that it is smaller than 6

Quote:

Originally Posted by MichaelLight

Prove that Attachment 22141.

How do i prove this? thank you...

Suppose otherwise, then by assumption:

$\sqrt{10}+2\sqrt{2}\ge 6$

Now square both sides and follow the consequences to get a contradiction.

CB
August 28th 2011, 01:11 PM
HallsofIvy

Re: Prove that it is smaller than 6

Quote:

Originally Posted by alexmahone

$\sqrt{10}+2\sqrt{2}<6$

$\Leftrightarrow 10+8+8\sqrt{5}<36$ (Squaring both sides)

$\Leftrightarrow 8\sqrt{5}<18$

$\Leftrightarrow 320<324$ (Squaring both sides)

which is true.

Normally, one would NOT start a proof by assuming what you want to prove, as alexmahone does here. However, this is a perfectly valid "synthetic proof". Every step is invertible. A "standard" proof would be given by going from "320< 324" upward.

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