Geometry and TrigonometryVery hardRadius of a cylinder as tall as it is wide
Geometry and Trigonometry practice question
A cylinder's height is equal to its diameter, and its volume is 54π cubic centimetres. What is the radius of the cylinder, in centimetres?
Answer: 3
With radius r, the height is 2r, so V = πr²(2r) = 2πr³ = 54π. Then r³ = 27 and r = 3.
Common wrong answers
- If you answered 6
- 6 is the height (and diameter), not the radius.
- If you answered 27
- 27 is r³.
- If you answered 3.78
- 3.78 is the cube root of 54, forgetting the factor 2.
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