Triangle Cookie Company

Geometry Level 4

The Triangle Cookie Company in the historic district of the Old Town has been making its perfectly round cookies for decades and selling them in their signature isosceles triangle boxes, with three rows separated by straight red packing strips, following these two packaging rules:

  1. All of the cookies are exactly the same size.
  2. All of the cookies in the same row are tangent to each other, to the red strips, and to the sides of the boxes, with no gaps.

An enterprising employee decides to try improving the packing efficiency--measured by the ratio of the area occupied by the cookies to the area of the box--by relaxing the first rule to "All of the cookies in the same row are exactly the same size." This means that each row could have cookies of a different diameter than the other 2 2 rows. The cookies will be packaged in an isosceles triangle box with suitable dimensions, so that it obeys the second rule.

Let R R be the packing efficiency. Then, approximately, what's the maximum possible percentage increase in R R obtained by adjusting the sizes of the round cookies instead of keeping them all the same?

12.5 % 12.5 \% 2.5 % 2.5 \% 0.5 % 0.5 \% 0.1 % 0.1 \% 0.02 % 0.02 \% 0.004 % 0.004 \%

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1 solution

Michael Mendrin
Mar 29, 2017

Let s s be the slope of the sides of the triangle, and let r 3 = 1 , r 2 , r 1 r_3=1, r_2, r_1 be the radii of the cookies, with the bottom row being all unit circles. The length of the base and the area of the triangle are

b = 2 ( 2 + 1 s ( 1 + 1 + s 2 ) ) b=2\left( 2+\dfrac { 1 }{ s } \left( 1+\sqrt { 1+{ s }^{ 2 } } \right) \right)

A = 1 2 b ( 1 2 b s ) A=\frac { 1 }{ 2 } b\left( \dfrac { 1 }{ 2 } bs \right)

The radii then works out to

r 3 = 1 r_3=1

r 2 = ( 2 + 1 s ( 1 + 1 + s 2 ) ) ( 1 + 1 s ( 1 + 1 + s 2 ) ) r 3 { r }_{ 2 }=\dfrac { \left( 2+\dfrac { 1 }{ s } \left( -1+\sqrt { 1+{ s }^{ 2 } } \right) \right) }{ \left( 1+\dfrac { 1 }{ s } \left( 1+\sqrt { 1+{ s }^{ 2 } } \right) \right) } { r }_{ 3 }

r 1 = ( 1 + 1 s ( 1 + 1 + s 2 ) ) ( 0 + 1 s ( 1 + 1 + s 2 ) ) r 2 { r }_{ 1 }=\dfrac { \left( 1+\dfrac { 1 }{ s } \left( -1+\sqrt { 1+{ s }^{ 2 } } \right) \right) }{ \left( 0+\dfrac { 1 }{ s } \left( 1+\sqrt { 1+{ s }^{ 2 } } \right) \right) } { r }_{ 2 }

The ratio of area of cookies to the area of the triangle then becomes

R = π ( 3 r 3 2 + 2 r 2 2 + r 1 2 ) A R=\dfrac { \pi \left( 3{ { r }_{ 3 } }^{ 2 }+2{ { r }_{ 2 } }^{ 2 }+{ { r }_{ 1 } }^{ 2 } \right) }{ A }

With s s as the variable, by numerical means the maximum value for R R can be found to be

R = 0.7201310578 R=0.7201310578…
r 1 = 0.967197787 r_1=0.967197787…
r 2 = 0.987327667 r_2=0.987327667…
r 3 = 1 r_3=1
s = 1.935156 s=1.935156…

while in the original case it was

R = 0.7199889689 R=0.7199889689…
r 1 = 1 r_1=1
r 2 = 1 r_2=1
r 3 = 1 r_3=1
s = 2 s=2

so that the percent improvement is

100 ( 0.7201310578 0.7199889689 0.7199889689 ) = 0.0197349... % 0.02 % 100 \left( \dfrac {0.7201310578…-0.7199889689… } {0.7199889689…} \right)=0.0197349...\% \; \simeq 0.02\%

Can you please explain how to extrapolate the first two equations ? Apologizing for my incompetence.

Alessandro Mattia - 4 years, 1 month ago

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