Blue or Yellow plus Red

Geometry Level 3

The figure shows a yellow equilateral triangle, D E F DEF , inscribed inside a unit equilateral triangle, A B C ABC .

If the blue area is the same as the yellow area, what is the radius, R R , of the red incircles inscribed in the three subtriangles? If R = a b c d R = \dfrac{a\sqrt b - \sqrt c}{d} , submit a + b + c + d a+b+c+d .


The answer is 23.

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2 solutions

Chew-Seong Cheong
May 22, 2021

Since the yellow area and blue area are equal, the area of the yellow equilateral triangle is half that of the unit equilateral triangle, and the area of one of the blue triangle is 1 6 \dfrac 16 that of the unit equilateral triangle or A = 1 6 3 4 = 1 8 3 A = \dfrac 16 \cdot \dfrac {\sqrt 3}4 = \dfrac 1{8\sqrt 3} .

Since the yellow equilateral triangle is half that of the unit equilateral triangle, its side length is 1 2 \dfrac 1{\sqrt 2} . Let us consider one of the blue triangles B D E \triangle BDE . Then D E = 1 2 DE = \dfrac 1{\sqrt 2} . Let B E = a BE = a ; then A D = a AD = a and D B = 1 a DB = 1-a . The semiperimeter s = 1 2 + a + 1 a 2 = 1 2 + 1 2 s = \dfrac {\frac 1{\sqrt 2} + a + 1 - a}2 = \dfrac {\frac 1{\sqrt 2} + 1}2 .

The radius of a red circle

r = A s = 1 4 3 ( 1 2 + 1 ) = 2 3 ( 2 1 ) 4 3 ( 2 + 1 ) ( 2 1 ) = 2 3 6 12 r = \frac As = \frac 1{4\sqrt 3\left(\frac 1{\sqrt 2}+1\right)} = \frac {\sqrt 2 \cdot \sqrt 3(\sqrt 2-1)}{4\cdot 3 (\sqrt 2+ 1)(\sqrt 2 - 1)} = \frac {2\sqrt 3 - \sqrt 6}{12}

The required answer is a + b + c + d = 2 + 3 + 6 + 12 = 23 a+b+c+d = 2 + 3 + 6 + 12 = \boxed{23} .

Saya Suka
May 22, 2021

Yellow area : yellow-blue area = 1 : 2
Yellow side : yellow-blue side = 1 : √2
Therefore, FD = (1 / √2) × AC = 1 / √2

AD = FC, so
One blue perimeter
= FD + DA + AF
= FD + CF + AF
= FD + (CF + FA)
= FD + CA
= 1/√2 + 1

One blue area
= (1 / 3) × (1 / 2) × ABC area
= (1 / 6) × (1² × sin 60° / 2)
= √3 / 24
= (1 / 2) × (One blue perimeter) × R
= (1 + √2)R / 2√2

R = 2√6 / 24(1 + √2)
= (√2 – 1) / 2√6
= (√12 – √6) / 12
= (2√3 – √6) / 12

Answer = 2 + 3 + 6 + 12 = 23

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