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Arc length help

rock.freak667
Homework Helper
6,230
31
1.Homework Statement

[tex]y=x^{\frac}{1}{2}}- \frac{1}{3}x^{\frac{3}{2}}+\lambda[/tex]

For [itex]0 \leq x \leq 3[/itex]
Show that the arc length,s=[itex]2\sqrt{3}[/itex]
2. Homework Equations
[tex]s=\int_{x_1} ^{x_2} \sqrt{1+ (\frac{dy}{dx})^2} dx[/tex]


3. The Attempt at a Solution


[tex]\frac{dy}{dx}=\frac{1}{2\sqrt{x}} - \frac{1}{2}x^{\frac{1}{2}}[/tex]

[tex] (\frac{dy}{dx})^2=\frac{1}{4x}-\frac{1}{2}+\frac{x}{2}[/tex]

[tex]s=\int_{0} ^{3} \sqrt{\frac{1}{4x}+\frac{x}{2}+\frac{1}{2}} dx[/tex]

and now that integral seems a bit too odd to get the answer [itex]2\sqrt{3}[/itex]
 

Answers and Replies

Gib Z
Homework Helper
3,344
4
You made a tiny mistake when you squared the derivative, the coefficient of the linear term should be 1/4.

You integral, when corrected, then becomes

[tex]s= \int^3_0 \sqrt{ \frac{1}{4x} + \frac{x}{4} + \frac{1}{2} } dx[/tex]

[tex]= \frac{1}{2} \int^3_0 2\sqrt{ \frac{1}{4x} + \frac{x}{4} + \frac{1}{2} } dx[/tex]

[tex]= \frac{1}{2} \int^3_0 \sqrt{x + 2 + \frac{1}{x} } dx[/tex]

I think I've already given out too much =]. Now just ignore that the bottom limit of the integral is x=0, or be rigorous and take limits instead.
 
It might also be a good idea to think about completing the square ...
 
rock.freak667
Homework Helper
6,230
31
It might also be a good idea to think about completing the square ...
I was trying to do that but I kept on squaring it wrongly...even though I squared it out like 9 times...Seems I have to keep my eye out for those tiny mistakes...Thanks Gib Z & Pere Callahan!
 

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