ISEE Upper Level Quantitative : Spheres

Study concepts, example questions & explanations for ISEE Upper Level Quantitative

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Example Questions

Example Question #42 : Solid Geometry

\displaystyle t is a positive number. Which is the greater quantity?

(A) The surface area of a sphere with radius \displaystyle t

(B) The surface area of a cube with edges of length \displaystyle 2t

Possible Answers:

(B) is greater

It is impossible to determine which is greater from the information given

(A) is greater

(A) and (B) are equal 

Correct answer:

(B) is greater

Explanation:

The surface area of a sphere is \displaystyle 4 \pi times the square of its radius, which here is \displaystyle t; the surface area of the sphere in (A) is \displaystyle 4 \pi t^{2}.

The area of one face of a cube is the square of the length of an edge, which here is \displaystyle 2t, so the area of one face of the cube in (B) is \displaystyle (2t) ^{2} = 4t^{2}. The cube has six faces so the total surface area is \displaystyle 6 \cdot 4t^{2} = 24t^{2}.

\displaystyle \pi < 4, so \displaystyle 4 \pi t^{2} < 16 t^{2} < 24 t^{2}, giving the sphere less surface area. (B) is greater.

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