Appearances are sometimes deceiving

Geometry Level 4

A triangle's angles are: 40 ° 40° , 60 ° 60° , and 80 ° 80° . We drop perpendiculars from the midpoints of the sides as is shown the figure below.

If the area of the blue hexagon is t t , and the area of the triangle is T T , then find the value of t T \dfrac{t}{T} .

4 7 \dfrac{4}{7} 1 3 \dfrac{1}{3} 3 5 \dfrac{3}{5} 2 3 \dfrac{2}{3} None of the others. 1 2 \dfrac{1}{2} 5 9 \dfrac{5}{9} 3 4 \dfrac{3}{4}

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1 solution

Maria Kozlowska
Jun 29, 2017

Segments D E , E F , D F DE,EF,DF divide A B C \triangle ABC into four congruent triangles. Points G , H , I G,H,I area orthocentres of triangles E F A , E D C , D F B EFA, EDC, DFB respectively. Hexagon E H D I F G EHDIFG can be divided into four triangles: D E F , E F G , E D H , D F I DEF, EFG, EDH, DFI . Area of D E F \triangle DEF is 1 4 \frac{1}{4} of the area of A B C \triangle ABC . Triangles E F G , E D H , D F I EFG, EDH, DFI make up the area of whole congruent triangle i.e. E F A EFA as their vertices are made up of orthocentre and each pair of the reference triangle sides. This means that hexagon area is equal to areas of two smaller triangles like that of D E F DEF and E F A EFA which in turn is half of the area of triangle A B C ABC . The answer is 1 2 \boxed{\frac{1}{2}} .

Note:

1) All vertices of the hexagon lie on the nine-point circle of the large triangle.

2) The ratio is the same regardless of the angle measures.

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