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.
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Let r 1 be the top radius of the truncated cone, let r 2 be the bottom radius of the truncated cone, and let the radius of the sphere be 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 .
Now consider the right triangle shown below:
By the Pythagorean Theorem,
( r 2 − r 1 ) 2 + 2 2 r 2 2 − 2 r 1 r 2 + r 1 2 + 4 r 1 r 2 = ( r 1 + r 2 ) 2 = r 1 2 + 2 r 1 r 2 + r 2 2 = 1 ( 1 )
The volume of the sphere and truncated cone are, respectively:
V S V T = 3 4 π r 3 = 3 4 π = 3 1 π ( r 1 2 + r 1 r 2 + r 2 2 ) h = 3 2 π ( r 1 2 + r 1 r 2 + r 2 2 )
The volume of the truncated cone is twice the volume of the sphere. Therefore,
3 2 π ( r 1 2 + r 1 r 2 + r 2 2 ) r 1 2 + r 1 r 2 + r 2 2 = 3 8 π = 4 ( 2 )
Recall that 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 r 1 2 + 2 r 1 r 2 + r 2 2 ( r 1 + r 2 ) 2 r 1 + r 2 = 4 = 5 = 5 = 5
Alternatively, subtract 3 r 1 r 2 = 3 from both sides to complete the square:
r 1 2 − 2 r 1 r 2 + r 2 2 ( r 2 − r 1 ) 2 r 2 − r 1 = 1 = 1 = 1
Solving this system of equations gives:
r 1 r 2 = 2 5 − 1 = 2 5 + 1
Then the ratio of the larger radius to the smaller radius is:
r 1 r 2 = 5 − 1 5 + 1 ≈ 2 . 6 1 8