Second pair of triangles with equal sum of integer Torricelli-Ferma distanses

Let P P be a point strictly in the interior of a scalene triangle A B C ABC with integer side lengths, and let

f ( P ) = P A + P B + P C f(P) = PA + PB + PC

For each triangle A B C ABC , let h A B C = min P [ f ( P ) ] h_{ABC} = \min_P\big[f(P)\big] .

Now, consider all triangles for which P A , P B , P C PA, PB, PC are all integers. The first smallest value minimum value of h A B C = Q 1 = 16464 h_{ABC}=Q_1=16464 for which exist two integer triangles with equal sum of integer Torricheli-Ferma distanses

P A 11 + P B 11 + P C 11 = P A 12 + P B 12 + P C 12 = 16464 P A 11 = 6825 , P B 11 = 5544 , P C 11 = 4095 P A 12 = 7040 , P B 12 = 5200 , P C 12 = 4224 PA_{11}+PB_{11}+PC_{11}=PA_{12}+PB_{12}+PC_{12}=16464 \\ PA_{11}=6825, \quad PB_{11}=5544, \quad PC_{11}=4095 \\ PA_{12}=7040, \quad PB_{12}=5200, \quad PC_{12}=4224

What is the next (second) value of equal sum h A B C = Q 2 h_{ABC}=Q_2 that there exist two triangles

P A 21 + P B 21 + P C 21 = P A 22 + P B 22 + P C 22 = Q 2 ? PA_{21}+PB_{21}+PC_{21}=PA_{22}+PB_{22}+PC_{22}=Q_2?

Use a computer for your calculation. This problem is connect with P and that next triangle .


The answer is 20280.

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1 solution

David Vreken
Apr 26, 2020

Since a Torricelli-Fermat point P P has the property that A P B = A P C = B P C = 120 ° \angle APB = \angle APC = \angle BPC = 120° , then by the law of cosines:

A B 2 = P A 2 + P B 2 2 P A P B cos 120 ° = P A 2 + P A P B + P B 2 AB^2 = PA^2 + PB^2 - 2 \cdot PA \cdot PB \cdot \cos 120° = PA^2 + PA \cdot PB + PB^2

A C 2 = P A 2 + P C 2 2 P A P C cos 120 ° = P A 2 + P A P C + P C 2 AC^2 = PA^2 + PC^2 - 2 \cdot PA \cdot PC \cdot \cos 120° = PA^2 + PA \cdot PC + PC^2

B C 2 = P B 2 + P C 2 2 P B P C cos 120 ° = P B 2 + P B P C + P C 2 BC^2 = PB^2 + PC^2 - 2 \cdot PB \cdot PC \cdot \cos 120° = PB^2 + PB \cdot PC + PC^2

The below Python program finds all numbers a a and b b between 0 < a < b < 15000 0 < a < b < 15000 such that a 2 + a b + b 2 a^2 + ab + b^2 is a perfect square, then uses that list to find integer sets of ( P A , P B , P C ) (PA, PB, PC) that give integer triangle sides, and finally displays all h = P A + P B + P C h = PA + PB + PC with more than one possible ( P A , P B , P C ) (PA, PB, PC) set. The second h h value computes to 20280 \boxed{20280} .

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# Second Pair of Triangles with Equal Sum of Integer Torricelli-Fermat Distances
# Python

# set up variables
# f[a][b] is a list of numbers such that a ** 2 + a * b + b ** 2 is a perfect square.
#   For example, f[5] = [3, 16] because 5 ** 2 + 5 * 3 + 3 ** 2 is a perfect square and
#   5 ** 2 + 5 * 16 + 16 ** 2 is a perfect square.
# g[h][(PA, PB, PC)] is a list of numbers such that PA and PB, PB and PC, and PA and PC are in f and
#   h = PA + PB + PC. For example, g[16464] = [(4095, 5544, 6825), (4224, 5200, 7040)]
import math
fmax = 15000
gmax = 3 * fmax
f = []
for a in range(0, fmax):
    f.append([])
g = []
for a in range(0, gmax):
    g.append([])

# function to determine if x is a perfect square
def isSquare(x):
    s = int(math.sqrt(x))
    return (s ** 2 == x)

# populate f
for a in range(1, fmax):
    for b in range(a + 1, fmax):
        if isSquare(a ** 2 + a * b + b ** 2):
            f[a].append(b)
            f[b].append(a)

# populate g
for a in range(0, fmax):
    for b in range(0, len(f[a])):
        for c in range(0, len(f[f[a][b]])):
            for d in range(0, len(f[f[f[a][b]][c]])):
                PA = a
                PB = f[a][b]
                PC = f[f[a][b]][c]
                P4 = f[f[f[a][b]][c]][d]
                if PA == P4 and PA < PB and PB < PC:
                    h = PA + PB + PC
                    g[h].append((PA, PB, PC))

# display results
for a in range(0, gmax):
    if len(g[a]) > 1:
        print("h = " + str(a) + " for (PA, PB, PC) = " + str(g[a])[1:-1])

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h = 16464 for (PA, PB, PC) = (4095, 5544, 6825), (4224, 5200, 7040)
h = 20280 for (PA, PB, PC) = (1305, 4968, 14007), (6307, 6765, 7208)
h = 32928 for (PA, PB, PC) = (8190, 11088, 13650), (8448, 10400, 14080)

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