24) Show that the right circular cone of least curved surface and given volume has an altitude equal to \sqrt 2 time the radius of the base.

Answers (1)

Volume of cone(V)

 \frac{1}{3}\pi r^2h \Rightarrow h = \frac{3V}{\pi r^2}
curved surface area(A) = \pi r l
l^2 = r^2 + h^2\\ l = \sqrt{r^2+\frac{9V^2}{\pi^2r^4}}
A = \pi r \sqrt{r^2+\frac{9V^2}{\pi^2r^4}} = \pi r^2 \sqrt{1+\frac{9V^2}{\pi^2r^6}}

\frac{dA}{dr} = 2\pi r \sqrt{1+\frac{9V^2}{\pi^2r^6}}+ \pi r^2.\frac{1}{2\sqrt{1+\frac{9V^2}{\pi^2r^6}}}.\frac{(-6r^5)9V^2}{\pi^2r^7}\\ \frac{dA}{dr} = 0\\ 2\pi r \sqrt{1+\frac{9V^2}{\pi^2r^6}}+ \pi r^2.\frac{1}{2\sqrt{1+\frac{9V^2}{\pi^2r^6}}}.\frac{(-6)9V^2}{\pi^2r^7} = 0 \\ 2\pi^2r^6\left ( 1+\frac{9V^2}{\pi^2r^6} \right ) = {27V^2}\\ 2\pi^2r^6\left ( \frac{\pi^2r^6+9V^2}{\pi^2r^6} \right ) = {27V^2}\\ 2\pi^2r^6 + 18V^2 = 27V^2\\ 2\pi^2r^6 = 9V^2\\ r^6 = \frac{9V^2}{2\pi^2}
Now , we can clearly varify that
\frac{d^2A}{dr^2} > 0
when r^6 =\frac{9V^2}{2\pi^2}
Hence, r^6 =\frac{9V^2}{2\pi^2} is the point of minima
V = \frac{\sqrt2\pi r^3}{3}
h = \frac{3V}{\pi r^2} = \frac{3.\frac{\sqrt2\pi r^3}{3}}{\pi r^2} = \sqrt2 r
Hence proved that  the right circular cone of least curved surface and given volume has an altitude equal to \sqrt 2 time the radius of the base

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