Ball and Basket

Geometry Level 5

A beach ball is lodged inside an upside-down basket in the shape of a truncated right cone. The volume of the basket is twice that of the ball.

The radius at the top of the cone is strictly smaller than the radius at the bottom of the cone. What is the ratio of the bottom radius to the top radius?

Give your answer to 3 decimal places.


The answer is 2.618.

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3 solutions

Andy Hayes
Nov 11, 2016

Let r 1 r_1 be the top radius of the truncated cone, let r 2 r_2 be the bottom radius of the truncated cone, and let the radius of the sphere be 1. 1. Consider the cross section of the figure:

By drawing the radii to the tangents with the cross sectional trapezoid, two kites are formed (they can be proved to be kites using HL triangle congruence). Thus, the slant height of the truncated cone is r 1 + r 2 . r_1+r_2.

Now consider the right triangle shown below:

By the Pythagorean Theorem,

( r 2 r 1 ) 2 + 2 2 = ( r 1 + r 2 ) 2 r 2 2 2 r 1 r 2 + r 1 2 + 4 = r 1 2 + 2 r 1 r 2 + r 2 2 r 1 r 2 = 1 ( 1 ) \begin{aligned} (r_2-r_1)^2+2^2 &= (r_1+r_2)^2 \\ r_2^2-2r_1r_2+r_1^2+4 &= r_1^2+2r_1r_2+r_2^2 \\ r_1r_2 &= 1 \quad \quad \quad \quad (1) \end{aligned}

The volume of the sphere and truncated cone are, respectively:

V S = 4 3 π r 3 = 4 3 π V T = 1 3 π ( r 1 2 + r 1 r 2 + r 2 2 ) h = 2 3 π ( r 1 2 + r 1 r 2 + r 2 2 ) \begin{aligned} V_S &= \frac{4}{3}\pi r^3 \\ &= \frac{4}{3}\pi \\ \\ V_T &= \frac{1}{3}\pi (r_1^2+r_1 r_2+r_2^2)h \\ &= \frac{2}{3} \pi (r_1^2+r_1 r_2+r_2^2) \end{aligned}

The volume of the truncated cone is twice the volume of the sphere. Therefore,

2 3 π ( r 1 2 + r 1 r 2 + r 2 2 ) = 8 3 π r 1 2 + r 1 r 2 + r 2 2 = 4 ( 2 ) \begin{aligned} \frac{2}{3} \pi (r_1^2+r_1 r_2+r_2^2) &= \frac{8}{3}\pi \\ r_1^2+r_1 r_2+r_2^2 & =4 \quad \quad (2) \end{aligned}

Recall that r 1 r 2 = 1 , r_1 r_2=1, and add this to both sides of the equation to complete the square:

r 1 2 + r 1 r 2 + r 2 2 = 4 r 1 2 + 2 r 1 r 2 + r 2 2 = 5 ( r 1 + r 2 ) 2 = 5 r 1 + r 2 = 5 \begin{aligned} r_1^2+r_1r_2+r_2^2 &= 4 \\ r_1^2+2r_1r_2+r_2^2 &= 5 \\ (r_1+r_2)^2 &= 5 \\ r_1+r_2 &= \sqrt{5} \end{aligned}

Alternatively, subtract 3 r 1 r 2 = 3 3r_1r_2=3 from both sides to complete the square:

r 1 2 2 r 1 r 2 + r 2 2 = 1 ( r 2 r 1 ) 2 = 1 r 2 r 1 = 1 \begin{aligned} r_1^2-2r_1r_2+r_2^2 &= 1 \\ (r_2-r_1)^2 &= 1 \\ r_2-r_1 &= 1 \end{aligned}

Solving this system of equations gives:

r 1 = 5 1 2 r 2 = 5 + 1 2 \begin{aligned} r_1 &= \frac{\sqrt{5}-1}{2} \\ r_2 &= \frac{\sqrt{5}+1}{2} \end{aligned}

Then the ratio of the larger radius to the smaller radius is:

r 2 r 1 = 5 + 1 5 1 2.618 \frac{r_2}{r_1}=\frac{\sqrt{5}+1}{\sqrt{5}-1}\approx \boxed{2.618}

Why u assumed the radius of sphere 1 unit?

akt kriss - 4 years, 6 months ago

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If you gave it a variable name, then it would have given the same result. Since the problem asks for the ratio of lengths, one can assume that this ratio is constant regardless of what the ball's radius is. Therefore, I gave it a computationally convenient value.

Andy Hayes - 4 years, 6 months ago
Vishwash Kumar
Nov 23, 2016

http://artofproblemsolving.com/wiki/index.php?title=2014 AMC 10B Problems/Problem 23. This is the link you find the problem and it's solution. Some how the button for link is missing hence given the full URL.

Found it! http://artofproblemsolving.com/wiki/index.php?title=2014 AMC 10B Problems/Problem 23

W Rose - 4 years, 1 month ago

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