 Optimal aspect ratios of closed cylinders

A closed circular cylinder of radius $a>0$ and length $\ell >0$ has volume $\pi a^2\ell$ and total surface area $2\pi a(a+\ell )$, including both end disks. At fixed area $A$, Lagrange multipliers for maximizing the volume give $\ell =2a$, henceEquivalently, eliminate $\ell =A/(2\pi a)-a$ and maximize $(A/2)a-\pi a^3$ on $0<a<\sqrt{A/(2\pi )}$. The derivative changes from positive to negative at the displayed radius, and the volume tends to zero at both ends.

For a cylinder inscribed in a sphere of radius $R$, the constraint is $a^2+{\ell }^2/4=R^2$. Maximizing $a^2+a\ell$ by Lagrange multipliers gives ${\ell }^2+2a\ell =4a^2$, so the positive aspect ratio is $\ell /a=\sqrt{5}-1$. It is independent of $R$. With $q=\ell /a$, the area is $2\pi R^2(1+q)/(1+q^2/4)$; differentiating this expression confirms the unique maximum on $q>0$.