# Enigmatic Code

Programming Enigma Puzzles

## Enigma 1313: Triangles

From New Scientist #2471, 30th October 2004

Draw a triangle ABC.  On the side AB mark the point P such that AP=(2/5)AB, on BC mark Q such that BQ=(2/5)BC and on CA mark R such that CR=(2/5)CA.

Draw the lines AQ, BR and CP.  Call the point where AQ and BR cross X, the point where BR and CP cross Y and the point where CP and AQ cross Z.

If you did the appropriate calculations you would find that the area of triangle XYZ is 1/19 of the area of triangle ABC.

I went through the whole procedure above again, but, this time with each occurrence of the number 2/5 replaced by the number k, which is between 0 and 1/2.  This time I found that the area of triangle XYZ was 1/37 of the area of triangle ABC.

What was the number k?

[enigma1313]

### 6 responses to “Enigma 1313: Triangles”

1. Jim Randell 29 June 2014 at 8:11 am

This Python program uses the SymPy to do the maths. It runs in 444ms.

from sympy import symbols, sqrt, sin, pi, simplify, Rational, Eq, solve
from enigma import printf

def degrees(d):
return pi * d / 180

# as the procedure works with any triangle we will consider the case
# where ABC is an equilateral triangle with sides of unit length
#
# we're interested in the ratio of the areas of XYZ and ABC
#
# R = area(XYZ) / area(ABC)
#
# now:
#
# area(XYZ) = area(ABC) - 3 area(ABX)
#
# so:
#
# R = 1 - 3 area(ABX)/area(ABC)
#
# and:
#
# area(ABC) = sqrt(3) / 4

ABC = sqrt(3) / 4
printf("area(ABC) = {ABC}")

# now:
#
# area(ABX) = area(ABQ) - area(BQX)
#
# and:
#
# area(ABQ) = (1/2) AB BQ sin(angle(ABQ))

k = symbols('k')
ABQ = k * sin(degrees(60)) / 2
printf("area(ABQ) = {ABQ}")

# to calculate area(BQX) we consider the line RD parallel to AQ which
# creates a triangle BDR similar to BQX.
#
# area(BQX) = r^2 area(BDR), where r is the ratio of the side lengths
# of the similar triangles [*]
#
# r = BQ / BD
#
# and:
#
# BQ = k, BD = BC - DC, BC = 1
#
# to calculate DC consider the similar triangles RDC and AQC
#
# DC/QC = RC/AC
#
# and QC = 1 - k, RC = k, AC = 1, so:
#
# DC = k(1 - k) = k - k^2
#
# hence:
#
# BD = BC - DC = 1 - k + k^2
#
# so:
#
# r = k / (1 - k + k^2)

r = k / (1 - k + k ** 2)
printf("r = {r}")

# now:
#
# area(RDC) = (1/2) RC DC sin(angle(RCD))
#           = (1/2) k  (BC - BD) sin(60 degrees)
#
# BC = 1, BD = 1 - k + k ^ 2

RDC = simplify(k * (1 - (1 - k + k ** 2)) * sin(degrees(60)) / 2)
printf("area(RDC) = {RDC}")

# and:
#
# area(BDR) = area(BCR) - area(RDC)

BDR = simplify(ABQ - RDC)
printf("area(BDR) = {BDR}")

# and so area(BQX) [*]

BQX = simplify((r ** 2) * BDR)
printf("area(BQX) = {BQX}")

# and finally area(ABX)

ABX = simplify(ABQ - BQX)
printf("area(ABX) = {ABX}")

# so for R

R = simplify(1 - 3 * ABX / ABC)
printf("R = {R}")

# check known points:
# k = 0 or k = 1 => R = 1, k = 1/2 => R = 0, k = 2/5 => R = 1/19
for (k0, R0) in ((0, 1), (1, 1), (Rational(1, 2), 0), (Rational(2, 5), Rational(1, 19))):
Rs = R.subs({k: k0})
printf("k={k0} => R={Rs}")
assert Rs == R0

# and solve R = 1/37 for k
for s in solve(Eq(R, Rational(1, 37)), k):
if s.is_real:
printf("R=1/37 => k={s} {S}", S=('[SOLUTION]' if 0 < s < Rational(1, 2) else ''))


Solution: k = 3/7.

Here’s a diagram that explains how the maths works:

I’ve used an equilateral triangle, but a similar approach will work with any triangle and give the same result.

The equation derived for the ratio of the areas is:

$R = \frac{3k\left( k - 1 \right)}{k^2 - k + 1} + 1$

which, as can be seen in this graph, starts at R=1 when k=0, drops to R=0 at k=½ and then rises back to R=1 at k=1, which is how we would expect it to behave.

2. Jim Randell 29 June 2014 at 8:21 am

And here’s a constructive numerical solution in Python using the [[ find_value() ]] function from the enigma.py library. It uses a simple right angled triangle for the construction (although any triangle (A, B, C) could be specified in the calculation of R). After the numerical calculation it finds the closest rational approximation with a denominator of 1000 or less to give as the answer. It runs in 46ms.

from fractions import Fraction
from enigma import sqrt, find_value, printf

# tolerance
t = 1e-9

# area of a triangle defined by three points
def area((x1, y1), (x2, y2), (x3, y3)):
return abs(x1 * (y2 - y3) + x2 * (y3 - y1) + x3 * (y1 - y2)) / 2.0

# a point a fraction of the way along a line
def fraction(f, (x1, y1), (x2, y2)):
return (x1 + f * (x2 - x1), y1 + f * (y2 - y1))

# where do two lines intersect?
def intersect((x1, y1), (x2, y2), (x3, y3), (x4, y4)):
x12 = x1 - x2
x34 = x3 - x4
y12 = y1 - y2
y34 = y3 - y4
a = x1 * y2 - y1 * x2
b = x3 * y4 - y3 * x4
d = x12 * y34 - y12 * x34
return ((a * x34 - x12 * b) / d, (a * y34 - y12 * b) / d)

# do the procedure
def R(k):

# place the corners of the large triangle
A = (0.0, 0.0)
B = (1.0, 0.0)
C = (0.0, 1.0)

# compute the area of ABC
ABC = area(A, B, C)

# P is k along AB, ...
P = fraction(k, A, B)
Q = fraction(k, B, C)
R = fraction(k, C, A)

# X is at the intersection of AQ and BR, ...
X = intersect(A, Q, B, R)
Y = intersect(B, R, C, P)
Z = intersect(A, Q, C, P)

# compute the area of XYZ
XYZ = area(X, Y, Z)

# return the ratio
return XYZ / ABC

# check known points (k=0, 1, 1/2, 2/5)
for (k0, R0) in [(0.0, 1.0), (1.0, 1.0), (0.5, 0.0), (2.0 / 5.0, 1.0 / 19.0)]:
r = R(k0)
assert abs(r - R0) < t

# and find k (between 0 and 1/2) when R = 1/37
r = find_value(R, 1.0 / 37.0, 0.0, 0.5, t=t)
# and find a rational approximation to the answer
k = Fraction.from_float(r.v).limit_denominator(1000)

printf("R = 1/37 (~ {r.fv}) at k ~ {k} (~ {r.v})")


This code uses the ability to specify pairs as the formal arguments in function definitions, which Python 2.7.6 doesn’t seem to have a problem with, but it doesn’t work in Python 3.4.0, which is a shame as I thought it was rather a neat (and useful) trick. (PEP 3113 [ http://legacy.python.org/dev/peps/pep-3113/ ] explains their removal in Python 3, and how to work around it ).

3. Jim Randell 21 August 2017 at 4:56 pm

This is a special case of Routh’s Theorem [ https://en.wikipedia.org/wiki/Routh%27s_theorem ], which states that the ratio of the area of the central triangle XYZ to the original triangle ABC, where the sides are divided in the ratios 1:x, 1:y, 1:z is given by the formula:

(xyz – 1)² / ((xz + x + 1)(xy + y + 1)(yz + z + 1))

In this case the ratios are all the same x = y = z, which simplifies the formula to:

(x – 1)² / (x² + x + 1) = 1 – 3x / (x² + x + 1)

And for this problem the ratio is equal to 1/37

1 – 3x / (x² + x + 1) = 1/37
12/37 = x / (x² + x + 1)
12x² + 12x + 12 = 37x
12x² – 25x + 12 = 0
(3x – 4)(4x – 3) = 0

So each side is split in the ratio 3:4, hence the value of k we are looking for is 3/7.

A detailed determination of the various areas of the sub-divisions of the triangle is given in this document: [ https://enigmaticcode.files.wordpress.com/2018/01/rouths-theorem1.pdf ].

4. geoffrounce 21 August 2017 at 10:11 pm

There is another special case – the one-seventh triangle, where the ratio of the two parts of one side is 2:1 in Routh’s formula, which I see is equivalent to k = 1/3 in your earlier formula for R.

If k = 1/3 in your formula for R, then the central area XYZ is 1/7 of the main area ABC.

In Routh’s formula , in this case, x = y = z = 2 (ie k = 1/3), so the ratio of the central area to the overall area is (2 * 2 * 2 – 1) ^2 / (4 + 2 + 1) ^ 3

ie 7^2 / 7^3 = 1/7

5. geoffrounce 22 August 2017 at 9:01 pm

The central triangle area being 1/7 of the main triangle area became known as Feynman’s Triangle, apparently proved over dinner once by Feynmann, the famous physicist.
See: http://journal.geometryexpressions.com/pdf/feinmansteiner.pdf
The paper examines other ratios of the sides of the triangle, including 1/4, 1/7 and 1/8, which gives a central triangle area of 1/2 of the main triangle area.
There is also an interesting example of construction of the vertices of the inner 1/7 triangle with integer coordinates:
https://math.stackexchange.com/questions/1131161/how-do-i-find-the-area-of-a-triangle-formed-by-cevians

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