Can an arc of a parabola inside a circle of radius 1 have a length greater than 4?
Solution
The answer is yes. Consider the arc of the parabola inside the circle , where we initially assume that . This intersects the circle in three points, and . We claim that for sufficiently large, the length of the parabolic arc between and is greater than , which implies the desired result by symmetry. We express using the usual formula for arclength:
\begin{align*}
L &= \int_0^{\sqrt{2A-1}/A} \sqrt{1+(2Ax)^2} \, dx \\
&= \frac{1}{2A} \int_0^{2\sqrt{2A-1}} \sqrt{1+x^2} \, dx \\
&= 2 + \frac{1}{2A} \left( \int_0^{2\sqrt{2A-1}}(\sqrt{1+x^2}-x)\,dx -2\right),
\end{align*}
where we have artificially introduced into the integrand in the last step. Now, for ,
since diverges, so does . Hence, for sufficiently large , we have , and hence .
Note: a numerical computation shows that one must take to obtain , and that the maximum value of is about , achieved for .