Question : If the curved surface area of a cylinder is 1386 sq cm and the height is 21 cm, what will be its radius? (Take $\pi=\frac{22}{7}$)
Option 1: 21 cm
Option 2: 5.25 cm
Option 3: 10.5 cm
Option 4: 15.75 cm
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Correct Answer: 10.5 cm
Solution :
Given: Curved surface area of a cylinder = 1386 sq cm
Height = 21 cm
Curved surface area of cylinder = $2\pi rh$
⇒ $2\pi rh = 1386$
⇒ $2×\frac{22}{7}×r×21=1386$
⇒ $r=\frac{1386×7}{2×22×21}$
= 10.5 cm
Hence, the correct answer is 10.5 cm.
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