Find the ratio of the altitude h to the base radius r of a right circular cylinder having the largest lateral surface area S, if the right circular cylinder is to be inscribed in a sphere of radius R.
Find the ratio of the altitude h to the base radius r of a right circular cylinder having the largest lateral surface area S, if the right circular cylinder is to be inscribed in a sphere of radius R.
$(2r)^2 + h^2 = (2R)^2$
$4r^2 + h^2 = 4R^2$
$h = \sqrt{4R^2 - 4r^2}$
$S = 2\pi rh$
$S = 2\pi r\sqrt{4R^2 - 4r^2}$
$\dfrac{dS}{dr} = 2\pi \left[ r \cdot \dfrac{-8r}{2\sqrt{4R^2 - 4r^2}} + \sqrt{4R^2 - 4r^2} \right] = 0$
$\sqrt{4R^2 - 4r^2} = \dfrac{8r^2}{2\sqrt{4R^2 - 4r^2}}$
$4R^2 - 4r^2 = 4r^2$
$R^2 = 2r^2$
$r^2 = \frac{1}{2}R^2$
$r = \frac{1}{\sqrt{2}}R$
$h = \sqrt{4R^2 - 4r^2}$
$h = \sqrt{4R^2 - 4(\frac{1}{2}R^2)}$
$h = \sqrt{2R^2}$
$h = \sqrt{2}R$
$\text{Required ratio} = \dfrac{h}{r}$
$\text{Required ratio} = \dfrac{\sqrt{2}R}{\frac{1}{\sqrt{2}}R}$
$\text{Required ratio} = 2$ answer
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