Answer:
[tex]r=1.05\ \text{ft}[/tex]
[tex]h=2.12\ \text{ft}[/tex]
[tex]20.91\ \text{ft}^3[/tex]
Step-by-step explanation:
r = Radius
h = Height
Volume of cylinder = [tex]7.35\ \text{ft}^3[/tex]
[tex]V=\pi r^2h\\\Rightarrow h=\dfrac{V}{\pi r^2}\\\Rightarrow h=\dfrac{7.35}{\pi r^2}[/tex]
Surface area is given by
[tex]A=2\pi r^2+2\pi rh\\\Rightarrow A=2\pi r^2+2\pi r\dfrac{7.35}{\pi r^2}\\\Rightarrow A=2\pi r^2+\dfrac{14.7}{r}[/tex]
Differentiating with respect to radius we get
[tex]\dfrac{dA}{dr}=4\pi r-\dfrac{14.7}{r^2}[/tex]
Equating with zero we get
[tex]0=4\pi r-\dfrac{14.7}{r^2}\\\Rightarrow 4\pi r=\dfrac{14.7}{r^2}\\\Rightarrow r^3=\dfrac{14.7}{4\pi}\\\Rightarrow r=(\dfrac{14.7}{4\pi})^{\dfrac{1}{3}}\\\Rightarrow r=1.05[/tex]
[tex]\dfrac{d^2A}{dr^2}=4\pi-2\times \dfrac{14.7}{r^3}\\\Rightarrow \dfrac{d^2A}{dr^2}=4\pi+2\times \dfrac{14.7}{1.05^3}\\\Rightarrow \dfrac{d^2A}{dr^2}=37.96>0[/tex]
So, the value of the function is minimum at [tex]r=1.05[/tex]
[tex]h=\dfrac{7.35}{\pi r^2}=\dfrac{7.35}{\pi 1.05^2}\\\Rightarrow h=2.12[/tex]
So, the radius and height which would minimize the surface area is 1.05 feet and 2.12 feet respectively.
Surface area
[tex]A=2\pi r^2+2\pi rh\\\Rightarrow A=2\pi \times 1.05^2+2\pi 1.05\times 2.12\\\Rightarrow A=20.91\ \text{ft}^3[/tex]
The minimum surface area is [tex]20.91\ \text{ft}^3[/tex].