Big Ideas Math Geometry, 2014
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Big Ideas Math Geometry, 2014 View details
1. Sample Spaces and Probability
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Exercise 25 Page 674

The requested probability is a geometric probability.

π6, or about 52 %

Practice makes perfect

We are given a sphere that fits inside a cube. The sphere touches each side of the cube. Let the radius of the sphere be r.

sphere that fits inside a cube

We will find the probability that a point chosen at random inside the cube is also inside the sphere. The desired probability is a geometric probability, so we need to find the volume of the sphere and the volume of the cube. Since the radius of the sphere is r, and since the sphere touches the each side of the cube, one side length of the cube is 2r — the diameter of the sphere.

Volume
Sphere 4/3π r^3
Cube (2r)^3= 8r^3
Now we will find the probability that a randomly chosen point from the volume of the cube is also within the volume of the sphere.
P(Point in sphere)=Volume of the Sphere/Volume of the Cube
P(Point in sphere)=4/3π r^3/8r^3
Simplify right-hand side
P(Point in sphere)=4/3π/8
P(Point in sphere)=4π/24
P(Point in sphere)=π/6
Rewrite as percent
P(Point in sphere)=0.52359...
P(Point in sphere)≈ 0.52
P(Point in sphere)≈ 52 %
The probability that a randomly chosen point is inside the sphere within the cube is π6, or about 52 %.