A
B
C
D
is a square with an area of
1
.
E
and
F
are the midpoints of
A
B
and
B
C
, respectively. If the area of the blue quadrilateral is
q
p
, where
p
and
q
are coprime positive integers, then find the value of
p
+
q
.
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Let us put the figure in x y -coordinate system, with origin at A . Then the coordinates of the points are A = ( 0 , 0 ) , E = ( 2 1 , 0 ) , B = ( 1 , 0 ) , F = ( 1 , 2 1 ) , C = ( 1 , 1 ) , D = ( 0 , 1 ) Let G = A F ∩ D E and H = A F ∩ D B . The equations of segments A F , D E , D B are A F D E D B : y = 2 1 x : y = 1 − 2 x : y = 1 − x Therefore G H = A F ∩ D E = ( y = 2 1 x ) ∩ ( y = 1 − 2 x ) = ( 5 2 , 5 1 ) = A F ∩ D B = ( y = 2 1 x ) ∩ ( y = 1 − x ) = ( 3 2 , 3 1 ) Hence, by the shoelace method , the area of the blue quadrilateral (i.e. the area [ E B H G ] ) is given by [ E B H G ] = 2 1 [ 2 1 0 1 0 3 2 3 1 5 2 5 1 ] = 2 1 ( 2 1 × 0 + 1 × 3 1 + 3 2 × 5 1 + 5 2 × 0 − 0 × 1 − 0 × 3 2 − 3 1 × 5 2 − 5 1 × 2 1 ) = 6 0 7 Therefore, p + q = 7 + 6 0 = 6 7
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