Pumpkin Volume 12

Calculus Level 4

Dedicated to my grandson Fedor.

The parametric equations of a pumpkin surface with n n ridges are given to be:

X ( p , t , n ) = R sin ( p ) ( n + 1 ) cos ( t ) cos ( t ( n + 1 ) ) n Y ( p , t , n ) = R sin ( p ) ( n + 1 ) sin ( t ) sin ( t ( n + 1 ) ) n Z ( p , t , n ) = h cos ( p ) , \begin{array} { r c l } X(p,t,n)&=& R\sin{(p)}\frac {(n+1)\cos{(t)}-\cos{(t(n+1))}}{n} \\ Y(p,t,n) &=& R\sin{(p)} \frac {(n+1)\sin{(t)}-\sin{(t(n+1))}}{n} \\ Z(p,t,n) &=& h \cos (p), \end{array} where 0 t 2 π 0 \leq t \leq 2\pi and 0 p π 0\leq p \leq \pi .

Define V p ( R , h , n ) V_p(R,h,n) as the pumpkin's volume, and V s ( R ) = 4 3 π R 3 V_s(R) = \frac{4}{3} \pi R^3 .

If the value of V p ( R , R , 12 ) V s ( R ) \frac{V_p(R,R,12)}{V_s(R)} can be expressed as A B \frac AB , where A A and B B are coprime positive integers, submit your answer as A + B . A+B.

Geogebra 3d picture


The answer is 163.

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1 solution

Mark Hennings
Nov 2, 2020

With R = h R=h we have the parametrization r = ( r sin p ( n + 1 ) cos t cos ( ( n + 1 ) t ) n , r sin p ( n + 1 ) sin t sin ( ( n + 1 ) t ) n , r cos p ) \mathbf{r} \; = \; \left(r \sin p \frac{(n+1)\cos t - \cos((n+1)t)}{n} \,,\, r\sin p \frac{(n+1)\sin t - \sin((n+1)t)}{n}\,,\,r\cos p\right) and hence we have the elementary volume element ratio X ( r , p , t ) = r r ( r p × r t ) = 2 ( n + 1 ) ( n + 2 ) n 2 r 2 sin p sin 2 1 2 n t \begin{aligned} X(r,p,t) & =\; \left| \frac{\partial \mathbf{r}}{\partial r} \cdot \left(\frac{\partial \mathbf{r}}{\partial p} \times \frac{\partial \mathbf{r}}{\partial t}\right) \right| \\ & = \; \frac{2(n+1)(n+2)}{n^2}r^2 \sin p \sin^2 \tfrac12 nt \end{aligned} so the pumpkin volume is V p ( R , R , n ) = 0 R d r 0 π d p 0 2 π d t X ( r , p , t ) = 4 ( n + 1 ) ( n + 2 ) 3 n 2 π R 3 \begin{aligned} V_p(R,R,n) & = \; \int_0^R \,dr \int_0^\pi\,dp \int_0^{2\pi}\,dt \, X(r,p,t) \\ & = \; \frac{4(n+1)(n+2)}{3n^2} \pi R^3 \end{aligned} so that V p ( R , R , n ) V s ( R ) = ( n + 1 ) ( n + 2 ) n 2 \frac{V_p(R,R,n)}{V_s(R)} \; = \; \frac{(n+1)(n+2)}{n^2} This makes the correct answer 13 × 14 144 = 91 72 \tfrac{13 \times 14}{144} = \tfrac{91}{72} , so the desired answer is 163 \boxed{163} .

@Mark Hennings , we really liked your comment, and have converted it into a solution.

Brilliant Mathematics Staff - 7 months, 1 week ago

What does n represent here?

Vijay Simha - 7 months, 1 week ago

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The number of segments/ridges.

Mark Hennings - 7 months, 1 week ago

Where can I find more information about the elementary volume element ratio?

James Wilson - 7 months, 1 week ago

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A better name for it is the Jacobian ( x , y , z ) ( r , p , t ) \frac{\partial(x,y,z)}{\partial (r,p,t)}

Mark Hennings - 7 months, 1 week ago

I guess I need to review calculus.

James Wilson - 7 months, 1 week ago

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