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Find the exact length of the curve

shamieh

Active member
Sep 13, 2013
539
A little bit confused.

Find the exact length of the curve

\(\displaystyle y = \frac{1}{4}x^2 - \frac{1}{2}\ln x\)

\(\displaystyle 1 \le x \le 2\)

Using the formula: \(\displaystyle y = \sqrt{1 + (\frac{dy}{dx})^2} \, dx\)

I obtained this:

\(\displaystyle \int ^2_1 \sqrt{ \frac{1}{2} + \frac{x^2}{4} + \frac{1}{4x^2}}\)

Now my problem is I'm stuck. If I bring the \(\displaystyle \frac{1}{2}\) out I will have a \(\displaystyle \sqrt{\frac{1}{2}}\) which won't really do me any good. Any suggestions?
 

soroban

Well-known member
Feb 2, 2012
409
Hello, shamieh!

Find the exact length of the curve.

\(\displaystyle y \:=\: \tfrac{1}{4}x^2 - \tfrac{1}{2}\ln x,\;1 \le x \le 2\)

Using the formula: .\(\displaystyle y \:=\: \sqrt{1 + (\tfrac{dy}{dx})^2} \, dx\)

I obtained this: \(\displaystyle \int ^2_1 \sqrt{\tfrac{1}{2} + \tfrac{x^2}{4} + \tfrac{1}{4x^2}}\)

You're doing great!

Note that: .[tex]\tfrac{x^2}{4} + \tfrac{1}{2} + \tfrac{1}{4x^2} \;=\;\tfrac{1}{4}\left(x^2 + 2 + \tfrac{1}{x^2}\right) [/tex]

. . . . . . . [tex]=\;\tfrac{1}{4}\,\left(x + \tfrac{1}{x}\right)^2 [/tex]

Then: .[tex]\sqrt{\tfrac{1}{4}\left(x + \tfrac{1}{x}\right)^2} \;=\;\tfrac{1}{2}\left(x + \tfrac{1}{x}\right) [/tex]


And so you have: .[tex]\tfrac{1}{2}\int^2_1\left(x + \tfrac{1}{x}\right)\,dx[/tex]
 

shamieh

Active member
Sep 13, 2013
539
Hello, shamieh!


Note that: .[tex]\tfrac{x^2}{4} + \tfrac{1}{2} + \tfrac{1}{4x^2} \;=\;\tfrac{1}{4}\left(x^2 + 2 + \tfrac{1}{x^2}\right) [/tex]

. . . . . . . [tex]=\;\tfrac{1}{4}\,\left(x + \tfrac{1}{x}\right)^2 [/tex]

Then: .[tex]\sqrt{\tfrac{1}{4}\left(x + \tfrac{1}{x}\right)^2} \;=\;\tfrac{1}{2}\left(x + \tfrac{1}{x}\right) [/tex]


And so you have: .[tex]\tfrac{1}{2}\int^2_1\left(x + \tfrac{1}{x}\right)\,dx[/tex]
Thanks, I see what you're saying but I'm confused on the algebra. Here is what I am getting, maybe you can tell me where I am going wrong.

Ok, here is what I am getting now:

\(\displaystyle \frac{1}{4} (2 + x^2 + \frac{1}{x^2})\)

Oh WOW I'm an idiot. Stared at my paper for 40 minutes trying to figure out how you factored out a 1/4 before I realized that 1/4 * 2/1. Thank you so much for your help Sororban!
 
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shamieh

Active member
Sep 13, 2013
539
Oh I see now.
 
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