MathDB
Area of a Quadrilateral in a Square

Source:

January 18, 2009
geometryrectangleanalytic geometryratio

Problem Statement

In the figure, ABCD ABCD is a 2×2 2\times 2 square, E E is the midpoint of AD \overline{AD}, and F F is on BE \overline{BE}. If CF \overline{CF} is perpendicular to BE \overline{BE}, then the area of quadrilateral CDEF CDEF is [asy]defaultpen(linewidth(.8pt)); dotfactor=4;
pair A = (0,2); pair B = origin; pair C = (2,0); pair D = (2,2); pair E = midpoint(A--D); pair F = foot(C,B,E);
dot(A);dot(B);dot(C);dot(D);dot(E);dot(F);
label("AA",A,N);label("BB",B,S);label("CC",C,S);label("DD",D,N);label("EE",E,N);label("FF",F,NW);
draw(A--B--C--D--cycle); draw(B--E); draw(C--F); draw(rightanglemark(B,F,C,4));[/asy] (A)\ 2\qquad (B)\ 3 \minus{} \frac {\sqrt {3}}{2}\qquad (C)\ \frac {11}{5}\qquad (D)\ \sqrt {5}\qquad (E)\ \frac {9}{4}