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1980 AHSME Problems/Problem 21

Problem

[asy] size(150); defaultpen(linewidth(0.7)+fontsize(10)); pair B=origin, C=(15,3), D=(5,1), A=7*dir(72)*dir(B--C), E=midpoint(A--C), F=intersectionpoint(A--D, B--E); draw(E--B--A--C--B^^A--D); label("$A$", A, dir(D--A)); label("$B$", B, dir(E--B)); label("$C$", C, dir(0)); label("$D$", D, SE); label("$E$", E, N); label("$F$", F, dir(80));[/asy]

In triangle $ABC$, $\measuredangle CBA=72^\circ$, $E$ is the midpoint of side $AC$, and $D$ is a point on side $BC$ such that $2BD=DC$; $AD$ and $BE$ intersect at $F$. The ratio of the area of triangle $BDF$ to the area of quadrilateral $FDCE$ is

$\text{(A)} \ \frac 15 \qquad \text{(B)} \ \frac 14 \qquad \text{(C)} \ \frac 13 \qquad \text{(D)}\ \frac{2}{5}\qquad \text{(E)}\ \text{none of these}$

Solution

[asy] size(150); defaultpen(linewidth(0.7)+fontsize(10)); pair B=origin, C=(15,3), D=(5,1), A=7*dir(72)*dir(B--C), E=midpoint(A--C), F=intersectionpoint(A--D, B--E), M=midpoint(D--C); draw(E--B--A--C--B^^A--D); draw(E--M); label("$A$", A, dir(D--A)); label("$B$", B, dir(E--B)); label("$C$", C, dir(0)); label("$D$", D, SE); label("$E$", E, N); label("$F$", F, dir(80)); label("$M$", M, dir(SE)); [/asy]

Let $M$ be the midpoint of $\overline{DC}$. Then $\triangle ECM \sim \triangle ACD$ and $\overline{EM} \parallel \overline{AD}$. Since $\overline{EM} \parallel \overline{FD}$, it follows that $\triangle BFD \sim \triangle BEM$.

Let $a$ be the area of $\triangle BFD$. Since the sides of $\triangle BEM$ are twice as long as the corresponding sides of $\triangle BFD$, the area of $\triangle BEM$ must be $2^2=4$ times the area of $\triangle BFD$, that is, $4a$.

Since the height of $\triangle BEC$ is the same as the height of $\triangle BEM$ and the base of $\triangle BEC$ is $\frac{3}{2}$ times the base of $\triangle BEM$, the area of $\triangle BEC$ is $\frac{3}{2}$ times the area of $\triangle BEM$, or $\frac{3}{2} \cdot 4a = 6a$.

Thus the area of quadrilateral $FDCE$ is $6a - a = 5a$, so the ratio of the area of $\triangle BFD$ to the area of quadrilateral $FDCE$ is $\frac{a}{5a} = \boxed{(\textbf{A})\ \frac{1}{5}}$.

-j314andrews

See also

1980 AHSME (Problems • Answer Key • Resources)
Preceded by
Problem 20 		Followed by
Problem 22
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 • 26 • 27 • 28 • 29 • 30
All AHSME Problems and Solutions

These problems are copyrighted © by the Mathematical Association of America, as part of the American Mathematics Competitions. Error creating thumbnail: Unable to save thumbnail to destination

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