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Geometry Difficulty 7.7 National olympiad, round 2 Find the answer

Can an arc of a parabola inside a circle of radius 1 have a length greater than 4?

A number or a short expression. Spacing and $ signs are ignored.

Solution

The answer is yes. Consider the arc of the parabola y=Ax2y=Ax^2 inside the circle x2+(y1)2=1x^2+(y-1)^2 = 1, where we initially assume that A>1/2A > 1/2. This intersects the circle in three points, (0,0)(0,0) and (±2A1/A,(2A1)/A)(\pm \sqrt{2A-1}/A, (2A-1)/A). We claim that for AA sufficiently large, the length LL of the parabolic arc between (0,0)(0,0) and (2A1/A,(2A1)/A)(\sqrt{2A-1}/A, (2A-1)/A) is greater than 22, which implies the desired result by symmetry. We express LL 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 x-x into the integrand in the last step. Now, for x0x \geq 0,
1+x2x=11+x2+x>121+x212(x+1); \sqrt{1+x^2}-x = \frac{1}{\sqrt{1+x^2}+x} > \frac{1}{2\sqrt{1+x^2}} \geq \frac{1}{2(x+1)};
since 0dx/(2(x+1))\int_0^\infty dx/(2(x+1)) diverges, so does 0(1+x2x)dx\int_0^\infty (\sqrt{1+x^2}-x)\,dx. Hence, for sufficiently large AA, we have 022A1(1+x2x)dx>2\int_0^{2\sqrt{2A-1}} (\sqrt{1+x^2}-x)\,dx > 2, and hence L>2L > 2.

Note: a numerical computation shows that one must take A>34.7A > 34.7 to obtain L>2L > 2, and that the maximum value of LL is about 4.00274.0027, achieved for A94.1A \approx 94.1.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.