9. If the surface area of a cylinder with radius of 4 feet is \( 48 \pi \) square feet, what is its volume? (A) \( 1 \pi \mathrm{ft}^{3} \) (B) \( 16 \pi \mathrm{ft}^{3} \) (C) \( 32 \pi \mathrm{ft}^{3} \) (D) \( 48 \pi \mathrm{ft.}^{3} \) (E) \( 64 \pi \mathrm{ft.}^{3} \)
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Bonus Knowledge
To find the volume of the cylinder, we first need to determine its height using the surface area formula. The formula for the surface area \( A \) of a cylinder is given by: \[ A = 2\pi r(h + r) \] Given that the radius \( r = 4 \) feet and the surface area \( A = 48\pi \) square feet, we can set up the equation: \[ 48\pi = 2\pi (4)(h + 4) \] Now we can simplify this: \[ 48 = 8(h + 4) \] Dividing both sides by 8: \[ 6 = h + 4 \] Solving for \( h \): \[ h = 2 \text{ feet} \] Now we can calculate the volume \( V \) of the cylinder using the formula: \[ V = \pi r^2 h \] Substituting in the values we have: \[ V = \pi (4^2)(2) = \pi (16)(2) = 32\pi \text{ cubic feet} \] So, the volume of the cylinder is \( 32 \pi \mathrm{ft}^{3} \), which corresponds to option (C).