Strange geo

Geometry Level 4

A B C D ABCD is a square with an area of 1 1 . E E and F F are the midpoints of A B AB and B C BC , respectively. If the area of the blue quadrilateral is p q \dfrac{p}{q} , where p p and q q are coprime positive integers, then find the value of p + q p+q .


The answer is 67.

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2 solutions

Ahmad Saad
Jun 27, 2017

Jaydee Lucero
Jun 30, 2017

Let us put the figure in x y xy -coordinate system, with origin at A A . Then the coordinates of the points are A = ( 0 , 0 ) , E = ( 1 2 , 0 ) , B = ( 1 , 0 ) , F = ( 1 , 1 2 ) , C = ( 1 , 1 ) , D = ( 0 , 1 ) A=(0,0),E=\left(\frac{1}{2},0\right),B=(1,0),F=\left(1,\frac{1}{2}\right),C=(1,1),D=(0,1) Let G = A F D E G=\overline{AF}\cap \overline{DE} and H = A F D B H=\overline{AF}\cap \overline{DB} . The equations of segments A F , D E , D B \overline{AF},\overline{DE},\overline{DB} are A F : y = 1 2 x D E : y = 1 2 x D B : y = 1 x \begin{aligned}\overline{AF} &: y=\frac{1}{2}x \\ \overline{DE} &: y = 1-2x \\ \overline{DB} &: y = 1-x \end{aligned} Therefore G = A F D E = ( y = 1 2 x ) ( y = 1 2 x ) = ( 2 5 , 1 5 ) H = A F D B = ( y = 1 2 x ) ( y = 1 x ) = ( 2 3 , 1 3 ) \begin{aligned} G &= \overline{AF}\cap \overline{DE} =\left ( y=\frac{1}{2}x \right) \cap (y = 1-2x) = \left( \frac{2}{5},\frac{1}{5} \right) \\ H &= \overline{AF}\cap \overline{DB} =\left ( y=\frac{1}{2}x \right) \cap (y = 1-x) = \left( \frac{2}{3},\frac{1}{3} \right)\end{aligned} Hence, by the shoelace method , the area of the blue quadrilateral (i.e. the area [ E B H G ] [EBHG] ) is given by [ E B H G ] = 1 2 [ 1 2 1 2 3 2 5 0 0 1 3 1 5 ] = 1 2 ( 1 2 × 0 + 1 × 1 3 + 2 3 × 1 5 + 2 5 × 0 0 × 1 0 × 2 3 1 3 × 2 5 1 5 × 1 2 ) = 7 60 [EBHG] = \frac{1}{2}\left [ \begin{array}{cccc} \frac{1}{2} & 1 & \frac{2}{3} & \frac{2}{5} \\ 0 & 0 & \frac{1}{3} & \frac{1}{5} \end{array} \right] = \frac{1}{2} \left( \frac{1}{2}\times 0 + 1\times \frac{1}{3} + \frac{2}{3}\times \frac{1}{5} + \frac{2}{5}\times 0 - 0 \times 1 - 0\times \frac{2}{3} - \frac{1}{3} \times \frac{2}{5} - \frac{1}{5}\times\frac{1}{2}\right) = \frac{7}{60} Therefore, p + q = 7 + 60 = 67 p+q=7+60=\boxed{67}

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