Arc Length integration always unusually difficult? Any special trick?

In summary, finding the arc length of a function can be difficult, even if you understand the formula and integration methods. A common approach is to solve for x first and then use the integration formula. For more complicated functions, using a u-substitution may be helpful. Some calculus books choose y to simplify the integral, but even simple functions like x^3 can result in non-elementary integrals. However, finding the arc length for a hyperbolic cosine curve is relatively easy. It is important to go slow and remember the rules when solving these types of problems.
  • #1
B3NR4Y
Gold Member
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I don't understand why finding the arc length is always difficult. I understand the formula and know pretty much all the integration methods, but whenever I try to find the arc length of a function like 8x2 = 27y3 from 1 to 8 it's unusually difficult.
I would start by solving for x
\begin{equation}
\begin{split}
&\frac{2}{3} x^{\frac{2}{3}} = y\\
&\frac{4}{9x^{\frac{1}{3}}} = y'\\
\end{split}
\end{equation}
Now I add this into the integration formula [itex]\int_a^b \sqrt{1+(y')^{2}} \, dx[/itex]
[tex]\begin{equation}
\begin{split}
L^{8}_{1} &= \int_1^8 \, \sqrt{1+ (\frac{4}{9x^{\frac{1}{3}}})^{2}} \, dx \\
&= \int_1^8 \, \sqrt{1+ (\frac{16}{81x^{\frac{2}{3}}})} \, dx \\
&= \int_1^8 \sqrt{\frac{81x^{\frac{2}{3}}+16}{81x^{\frac{2}{3}}}} \, dx \\
&= \int_1^8 \frac{\sqrt{81x^{\frac{2}{3}}+16}}{9x^{\frac{1}{3}}} \, dx
\end{split}
\end{equation}[/tex]
From here I would do a u-substitution of [itex]u=81x^{\frac{2}{3}}+16[/itex] and then take the derivative to find [itex]\frac{1}{54} du = \frac{1}{x^{\frac{1}{3}}} dx [/itex]
My new limits would be 97 and 340
\begin{equation}
\begin{split}
L^{8}_{1} &= \frac{1}{9*54} \int_{97}^{340} \sqrt{u} \, du \\
&= \frac{1}{486} \Bigg(\frac{3}{2} u^{\frac{3}{2}} |^{97}_{340} \Bigg) \\
&=7.28... \\
\end{split}
\end{equation}
I got the question right, by going slowly and typing it all out, derp. Anyway, is there a special trick for these types of problems? Or is it to just go slow and remember rules.
 
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  • #2
Those are often messy because of the form of the integrals.
Many calculus books choose y to make the integral simple.
Even simple y live x^3, sin(x), and 1/x give non-elementary integrals.
 
  • #3
Finding the arc length of the hyperbolic cosine curve is, however, fairly trivial.
:smile:
 

FAQ: Arc Length integration always unusually difficult? Any special trick?

1. Why is arc length integration considered difficult?

Arc length integration can be challenging because it involves finding the length of a curve, which is a continuous function, rather than a straight line. This requires advanced mathematical concepts and techniques, such as derivatives and integrals, which can be difficult to understand and apply.

2. What makes arc length integration different from regular integration?

Regular integration involves finding the area under a curve, while arc length integration involves finding the length of a curve. This difference in the objective of the integration process requires different techniques and formulas to be used, making arc length integration more complex.

3. Are there any special tricks for solving arc length integration problems?

Yes, there are some techniques that can make arc length integration easier. One such technique is using substitution to simplify the integral before solving it. Another trick is to break the curve into smaller sections and solve for the arc length of each section separately before adding them together.

4. What is the most common mistake made when solving arc length integration problems?

The most frequent mistake made in arc length integration is forgetting to take the square root of the integrand before integrating. This is because the formula for arc length involves taking the square root of the sum of the squares of the derivatives, which is often overlooked.

5. How can I improve my skills in arc length integration?

The best way to improve your skills in arc length integration is to practice solving different types of problems and familiarize yourself with the various techniques and formulas used. It is also helpful to seek guidance from a math tutor or professor who can provide personalized instruction and help clarify any doubts or difficulties you may have.

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