Question : The height of a cylinder is $\frac{2}{3}$rd of its diameter. Its volume is equal to the volume of a sphere whose radius is 4 cm. What is the curved surface area (in cm2) of the cylinder?
Option 1: $\frac{112}{3} \pi$
Option 2: $32 \pi$
Option 3: $\frac{128}{3} \pi$
Option 4: $40 \pi$
Correct Answer: $\frac{128}{3} \pi$
Solution : Given, The height of a cylinder is $\frac23$rd of its diameter. Cylinder volume is equal to the volume of a sphere whose radius is 4 cm. We know, Volume of cylinder = $\pi r^2h$ Volume of sphere = $\frac43\pi R^3$ Curved surface area of cylinder = $2\pi rh$ Where, $r$ = radius of the cylinder and $R$ = Radius of the sphere Let the radius of the cylinder be $3x$ ⇒ Diameter of cylinder = $2 × 3x$ = $6x$ Height of the cylinder = $6x × \frac23$ = $4x$ The radius of sphere $R$ = 4 cm According to the question, $\pi r^2h =\frac43\pi R^3$ ⇒ $3x × 3x × 4x = \frac43 × 4 × 4 × 4$ ⇒ $x^3 = \frac{4 × 4 × 4}{3 × 3 × 3}$ ⇒ $x = \sqrt[3]{\frac{4 × 4 × 4}{3 × 3 × 3}}$ ⇒ $x=\frac43$ Curved surface area of cylinder = $2 \pi rh$ = $2π × 3x × 4x$ = $2π × 3 × \frac43 × 4 × \frac43$ = $\frac{128}{3} \pi$ Hence, the correct answer is $\frac{128}{3}\pi$.
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