ACT Math · Area and volume

A cylinder has radius 4 and height 9. Which value is closest to its total surface area?

A cylinder has radius 4 and height 9. Which value is closest to its total surface area?

Answer and explanation

Answer: D: 327

The total surface area of a closed cylinder includes two circular bases and the curved side: 2πr2+2πrh=2π(16)+2π(4)(9)=32π+72π=104π≈326.72\pi r^2+2\pi rh=2\pi(16)+2\pi(4)(9)=32\pi+72\pi=104\pi\approx326.7, choice D. Choice A (226226) is the lateral surface only, 72π72\pi, and leaves out both bases. Choice B (276276) is 88π88\pi, which includes only one base. Choice C (452452) is 144π144\pi, which adds more area than the cylinder has. The question asks for the total surface, so count both bases. Sketching the net of a cylinder helps.

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