A special triangle corrected

Geometry Level pending

In a triangle A B C ABC as shown in the above diagram with B C = 1 BC=1 , a rectangle C D E F CDEF is formed where D D is the midpoint of B C BC , point E E lies on A B AB and point F F lies inside triangle A B C ABC . Interestingly, rectangle C D E F CDEF turns out to be a square.

Next another rectangle G H I J GHIJ is constructed such that point G G is where line E F EF extended meets A C AC , H H is another point on E F EF such that H F = F G HF=FG , I I lies on A B AB and J J lies inside triangle A B C ABC . Even more interestingly, rectangle G H I J GHIJ also turns out to be a square.

Now determine the reciprocal of the area of triangle C F G CFG .


The answer is 24.

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3 solutions

Marta Reece
Jul 7, 2017

C F = D C = 1 2 CF=DC=\frac12

Let side of small square be I H = a IH=a

Ratio E D B D = 1 \frac{ED}{BD}=1 , therefore I H H E = 1 \frac{IH}{HE}=1 and H E = a HE=a

H F = F G = a 2 HF=FG=\frac a2

E F = E H + H F = a + a 2 = 3 a 2 EF=EH+HF=a+\frac a2=\frac{3a}2

But we know that E F = 1 2 EF=\frac12 , that gives us the equation for a a as: 3 a 2 = 1 2 \frac{3a}2=\frac12 and a = 1 3 a=\frac13

F G = a 2 = 1 6 FG=\frac a2=\frac16

Area [ C F G ] = 1 2 × C F × F G = 1 2 × 1 2 × 1 6 = 1 24 [CFG]=\frac12\times CF\times FG=\frac12\times\frac12\times\frac16=\frac1{24}

Inverse of the area is: 24 \boxed{24}

Noel Lo
Jul 7, 2017

Considering that B C = 1 BC=1 , with D D as the midpoint of B C BC , we have B D = C D = 1 2 BD=CD=\frac{1}{2} . Then C D E F CDEF being a square means D E = E F = C F = C D = 1 2 DE=EF=CF=CD=\frac{1}{2} . Let F G = x FG=x . Then H F = F G = x HF=FG=x as given in the question which means E H = E F H F = 1 2 x EH=EF-HF=\frac{1}{2}-x .

Now note that B D E BDE and E H I EHI are similar triangles as B D BD and E H EH are parallel and so are D E DE and H I HI . Therefore E H H I = B D D E = 1 \frac{EH}{HI}=\frac{BD}{DE}=1 which means H I = E H = 1 2 x HI=EH=\frac{1}{2}-x . Considering G H I J GHIJ is also a square, G H = H I = 1 2 x GH=HI=\frac{1}{2}-x . Moreover, G H = H F + F G = x + x = ( 1 + 1 ) x = 2 x GH=HF+FG=x+x=(1+1)x=2x . Therefore:

2 x = 1 2 x 2x=\frac{1}{2}-x

2 x + x = 1 2 2x+x=\frac{1}{2}

( 2 + 1 ) x = 1 2 (2+1)x=\frac{1}{2}

3 x = 1 2 3x=\frac{1}{2}

x = 1 3 × 2 x=\frac{1}{3\times2}

x = 1 6 x=\frac{1}{6}

Therefore, the area of C F G = 1 2 × C F × F G = 1 2 × 1 2 × 1 6 CFG=\frac{1}{2} \times CF \times FG=\frac{1}{2} \times \frac{1}{2} \times \frac{1}{6} , the reciprocal of which is 2 × 2 × 6 = 4 × 6 = 24 2\times 2\times 6=4\times 6=\boxed{24}

Ahmad Saad
Jul 7, 2017

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