Help understanding/evaluating line integral over a curve

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Homework Help Overview

The discussion revolves around evaluating a line integral over a specified curve, with the integral defined as \(\int_{c} xy\:ds\) where the curve is parameterized by \(x=t^{2}\) and \(y=2t\) for \(0\leq t\leq4\). Participants are exploring the correct application of the line integral formula and the implications of entering approximate versus exact answers in an online homework system.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the application of the line integral formula and question whether the integral setup is correct. There are inquiries about the validity of the computed answer and the potential issues with decimal precision in the context of online submissions.

Discussion Status

The discussion has led to some participants suggesting that the original poster's approach appears correct, while others have noted the importance of checking the required precision for answers in the online system. There is acknowledgment of the challenges posed by computer-mediated assessments.

Contextual Notes

Participants mention constraints related to online homework submissions, including the limited number of attempts allowed and the need for exact answers rather than approximate values.

Kaldanis
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Evaluate the line integral, where C is the given curve.

\int_{c} xy\:ds, when C: x=t^{2}, \ y=2t\ , \ 0\leq t\leq4

To solve this I should use the formula \int^{b}_{a} f(x(t),y(t))\sqrt{(\frac{dx}{dt})^{2}+(\frac{dy}{dt})^{2}}dt

This gives me \int^{4}_{0} (t^{2}2t)\sqrt{(2t)^{2}+(2)^{2}}dt

When I solve this integral I get around 860.3. I've had various friends solve it and also checked on my calculator and wolfram, all getting 860.3 each time. I've been told this answer is wrong which leads me to believe I'm using the formula incorrectly. Is my integral above correct? Is it wrong to just plain solve the integral from 0 to 4?

I appreciate any help
 
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Who tells you the answer is wrong?
 
It's for online homework that only accepts the correct answer. I can only enter answers so many times before the question is locked, so I want to be sure of the answer before I enter it again
 
Kaldanis said:
Evaluate the line integral, where C is the given curve.

\int_{c} xy\:ds, when C: x=t^{2}, \ y=2t\ , \ 0\leq t\leq4

To solve this I should use the formula \int^{b}_{a} f(x(t),y(t))\sqrt{(\frac{dx}{dt})^{2}+(\frac{dy}{dt})^{2}}dt

This gives me \int^{4}_{0} (t^{2}2t)\sqrt{(2t)^{2}+(2)^{2}}dt

When I solve this integral I get around 860.3. I've had various friends solve it and also checked on my calculator and wolfram, all getting 860.3 each time. I've been told this answer is wrong which leads me to believe I'm using the formula incorrectly. Is my integral above correct? Is it wrong to just plain solve the integral from 0 to 4?

I appreciate any help
How about entering the exact answer.

Is it possible to enter square roots, etc?
 
The thing that occurs to me is to check the decimal places it wants. 860.34?
If there is nothing to tell you (context - how many dp is usually wanted?) then you should tell the person who set the exercise (check with other students).

afaict, there is nothing wrong with the working.
 
looks ok to me! :smile:

(btw, i assume you used an algebraic substitution, and not a trig one? :wink:)
 
Thank you everyone, the problem was that I was entering an approximate decimal value. It accepted \frac{8}{15}(1+391\sqrt{17}) as the correct answer!
 
grrr... we hates computer mediated assessments oh we does!
 
Simon Bridge said:
grrr... we hates computer mediated assessments oh we does!

:smile:
 

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