Le Monde puzzle [#1002]

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For once and only because it is part of this competition, a geometric Le Monde mathematical puzzle:

Given both diagonals of lengths p=105 and q=116, what is the parallelogram with the largest area? and when the perimeter is furthermore constrained to be L=290?

This made me jump right away to the quadrilateral page on Wikipedia, which reminds us that the largest area occurs when the diagonals are orthogonal, in which case it is A=½pq. Only the angle between the diagonals matters. Imposing the perimeter in addition is not solved there, so I wrote an R code looking at all the integer solutions, based on one of the numerous formulae for the area, like


where s is the half-perimeter and a,b,c,d are the lengths of the four sides:

for (alpha in (1:500)/1000){
 for (beta in (1:999)/1000){
  for (teta in (1:9999)*pi/10000){
   if (abs(a+b+c+d-2*s)2*maxur){

This code returned an area of 4350, to compare with the optimal 6090 (which is recovered by the above R code when the diagonal lengths are identical and the perimeter is the one of the associated square). (As Jean-Louis Foulley pointed out to me, this area can be found directly by assuming the quadrilateral is a parallelogram.)

Filed under: Kids, R Tagged: geometry, Le Monde, mathematical puzzle, optimisation, R, Rmpfr

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