Odd Sangaku

Geometry Level 4

In the right A B C \triangle ABC with side lengths A B = 27 \overline{AB} = 27 and B C = 36 \overline{BC} = 36 , a square, a rectangle and two circles of the same radius are inscribed.

Input the sum of areas of the square and the rectangle as your answer.


The answer is 240.

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2 solutions

Chew-Seong Cheong
Jan 14, 2021

First we note that A B C \triangle ABC is a 3 3 - 4 4 - 5 5 right triangle (this makes things a lot easier).

Label the square D E F G DEFG and the rectangle G H I J GHIJ . Extend F G FG to meet B C BC at K K . Since the two circles are congruent and the three angles of D G H \triangle DGH and B F K \triangle BFK are the same, D G H \triangle DGH and B F K \triangle BFK are congruent (this also makes things a lot easier).

Let the side length of square D E F G DEFG be a a , then D G = B F = a DG=BF=a , D H = B K = 4 a 3 DH=BK=\dfrac {4a}3 , and G H = F K = 5 a 3 GH=FK= \dfrac {5a}3 . Let G J = b GJ=b . We note that G J K \triangle GJK and B F K \triangle BFK are similar (makes things easy again), then

G J G K = B F F K = 3 5 G J = 3 5 G K = 3 5 ( F K F G ) b = 3 5 ( 5 a 3 a ) = 2 a 5 \begin{aligned} \frac {GJ}{GK} & = \frac {BF}{FK} = \frac 35 \\ GJ & = \frac 35 \cdot GK = \frac 35 (FK - FG) \\ \implies b & = \frac 35 \left(\frac {5a}3 - a \right) = \frac {2a}5 \end{aligned}

We have:

A F + F B = A B A F + 3 5 F G + G J = A B 5 4 a + 3 5 a + b = 27 25 + 12 + 8 20 a = 27 45 20 a = 27 a = 12 \begin{aligned} AF+FB & = AB \\ AF + \frac 35 \cdot FG + GJ & = AB \\ \frac 54a + \frac 35 a + b & = 27 \\ \frac {25+12+8}{20}a & = 27 \\ \frac {45}{20}a & = 27 \\ \implies a & = 12 \end{aligned}

The sum of areas of the square and rectangle is a 2 + b G H = a 2 + 2 a 5 5 a 3 = 5 3 a 2 = 240 a^2 + b \cdot GH = a^2 + \dfrac {2a}5 \cdot \dfrac {5a}3 = \frac 53a^2 = \boxed{240}

Sathvik Acharya
Jan 13, 2021

Let B A C = α A C B = 90 α \angle BAC=\alpha\implies \angle ACB=90-\alpha and A E F = 90 α \angle AEF=90-\alpha . So, we have, A B C A F E A F = 27 k \triangle ABC\sim AFE \implies AF=27k and E F = 36 k EF=36k for some constant k > 0 k>0 . Also, A B = 27 , B C = 36 AB=27,\;BC=36 and C A = A B 2 + B C 2 = 45 CA=\sqrt{AB^2+BC^2}=45 .

Simple angle chasing gives, A E F = 90 α B E L = α B L E = 90 α \angle AEF=90-\alpha\implies \angle BEL=\alpha\implies \angle BLE=90-\alpha A C B = G I H = 90 α G H I = α \angle ACB=\angle GIH=90-\alpha\implies GHI=\alpha We conclude that, A F E H G I I J C \triangle AFE\sim \triangle HGI\sim \triangle IJC and H G I E B L \triangle HGI\cong \triangle EBL , since they have the same inradius. A F E H G I A F H G = F E G I 27 k 36 k = 36 k G I G I = 48 k H I = H G 2 + G I 2 = 60 k H G I I J C H G I J = H I I C 36 k I J = 60 k 45 111 k I J = 3 ( 45 111 k ) 5 H G I E B L B E = G H = 36 k \begin{aligned}\triangle AFE\sim \triangle HGI&\implies \frac{AF}{HG}=\frac{FE}{GI} \\ \\ &\implies \frac{27k}{36k}=\frac{36k}{GI} \\ \\ &\implies GI=48k \\ \\ &\implies HI=\sqrt{HG^2+GI^2}=60k \\ \\ \triangle HGI\sim \triangle IJC &\implies \frac{HG}{IJ}=\frac{HI}{IC} \\ \\ &\implies \frac{36k}{IJ}=\frac{60k}{45-111k} \\ \\ &\implies IJ=\frac{3(45-111k)}{5} \\ \\ \triangle HGI\cong \triangle EBL &\implies BE=GH=36k \end{aligned} In right-triangle A F E AFE , A E 2 = A F 2 + E F 2 ( 27 36 k ) 2 = ( 27 k ) 2 + ( 36 k ) 2 ( 3 4 k ) 2 = ( 3 k ) 2 + ( 4 k ) 2 \begin{aligned} AE^2&=AF^2+EF^2 \\ \\ (27-36k)^2&=(27k)^2+(36k)^2\\ \\ (3-4k)^2&=(3k)^2+(4k)^2 \end{aligned} Solving the above quadratic equation, gives us k = 1 3 k=\dfrac{1}{3} . Therefore, area [ E F G H ] = ( E F ) 2 = ( 36 k ) 2 = 1 2 2 = 144 area [ H I J K ] = H I I J = 60 k 3 ( 45 111 k ) 5 = 20 24 5 = 96 \begin{aligned} \text{area}[EFGH]&=(EF)^2=(36k)^2=12^2=144 \\ \\ \text{area}[HIJK]&=HI\cdot IJ=60k\cdot \frac{3(45-111k)}{5} =20\cdot \frac{24}{5}=96 \end{aligned} area [ E F G H ] + area [ H I J K ] = 240 \implies \text{area}[EFGH]+\text{area}[HIJK]=\boxed{240}

The areas of each of the two congruent circles = 16*pi

Vijay Simha - 4 months, 4 weeks ago

27x + 36 y = 972 { 27 x 36 = 972}

y = 12 - 0.75x

y = 1.3333x + 12

y = 1.3333x - 8

y = 4.8

x = 9.6

(x-4)^2 + (y-4)^2 = 16

(x-17.6)^2 + (y-8.8)^2 = 16

All the equations for the figure shown in the problem have been written above.

Area of the Square = 12 x 12 = 144

Area of the Rectangle = 4.8 x 20 = 96

Sum of the area of the square and rectangle = 144+96 = 240.

Vijay Simha - 4 months, 4 weeks ago

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