In a triangle A B C as shown in the above diagram with B C = 1 , a rectangle C D E F is formed where D is the midpoint of B C , point E lies on A B and point F lies inside triangle A B C . Interestingly, rectangle C D E F turns out to be a square.
Next another rectangle G H I J is constructed such that point G is where line E F extended meets A C , H is another point on E F such that H F = F G , I lies on A B and J lies inside triangle A B C . Even more interestingly, rectangle G H I J also turns out to be a square.
Now determine the reciprocal of the area of triangle C F G .
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Considering that B C = 1 , with D as the midpoint of B C , we have B D = C D = 2 1 . Then C D E F being a square means D E = E F = C F = C D = 2 1 . Let F G = x . Then H F = F G = x as given in the question which means E H = E F − H F = 2 1 − x .
Now note that B D E and E H I are similar triangles as B D and E H are parallel and so are D E and H I . Therefore H I E H = D E B D = 1 which means H I = E H = 2 1 − x . Considering G H I J is also a square, G H = H I = 2 1 − x . Moreover, G H = H F + F G = x + x = ( 1 + 1 ) x = 2 x . Therefore:
2 x = 2 1 − x
2 x + x = 2 1
( 2 + 1 ) x = 2 1
3 x = 2 1
x = 3 × 2 1
x = 6 1
Therefore, the area of C F G = 2 1 × C F × F G = 2 1 × 2 1 × 6 1 , the reciprocal of which is 2 × 2 × 6 = 4 × 6 = 2 4
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C F = D C = 2 1
Let side of small square be I H = a
Ratio B D E D = 1 , therefore H E I H = 1 and H E = a
H F = F G = 2 a
E F = E H + H F = a + 2 a = 2 3 a
But we know that E F = 2 1 , that gives us the equation for a as: 2 3 a = 2 1 and a = 3 1
F G = 2 a = 6 1
Area [ C F G ] = 2 1 × C F × F G = 2 1 × 2 1 × 6 1 = 2 4 1
Inverse of the area is: 2 4