Problem

Rectangle $ABCD$ has side lengths $AB=84$ and $AD=42$ . Point $M$ is the midpoint of $\overline{AD}$ , point $N$ is the trisection point of $\overline{AB}$ closer to $A$ , and point $O$ is the intersection of $\overline{CM}$ and $\overline{DN}$ . Point $P$ lies on the quadrilateral $BCON$ , and $\overline{BP}$ bisects the area of $BCON$ . Find the area of $\triangle CDP$ .

Solution

$[asy] pair A,B,C,D,M,n,O,P; A=(0,42);B=(84,42);C=(84,0);D=(0,0);M=(0,21);n=(28,42);O=(12,18);P=(32,13); fill(C--D--P--cycle,lightgray); draw(A--B--C--D--cycle); draw(C--M); draw(D--n); draw(B--P); draw(D--P); label("$A$",A,NW); label("$B$",B,NE); label("$C$",C,SE); label("$D$",D,SW); label("$M$",M,W); label("$N$",n,N); label("$O$",O,(-0.5,1)); label("$P$",P,N); dot(A); dot(B); dot(C); dot(D); dot(M); dot(n); dot(O); dot(P); label("28",(0,42)--(28,42),N); label("56",(28,42)--(84,42),N); [/asy]$ Impose a coordinate system on the diagram where point $D$ is the origin. Therefore $A=(0,42)$ , $B=(84,42)$ , $C=(84,0)$ , and $D=(0,0)$ . Because $M$ is a midpoint and $M$ is a trisection point, $M=(0,21)$ and $N=(28,42)$ . The equation for line $DN$ is $y=\frac{3}{2}x$ and the equation for line $CM$ is $\frac{1}{84}x+\frac{1}{21}y=1$ , so their intersection, point $O$ , is $(12,18)$ . Using the shoelace formula on quadrilateral $BCON$ , its area is $2184$ . Therefore the area of triangle $BCP$ is $\frac{2184}{2}$ and the distance from $P$ to line $PC$ is $52$ and its $x$ -coordinate is $32$ . Because $P$ lies on the equation $\frac{1}{84}x+\frac{1}{21}y=1$ , $P$ 's $y$ -coordinate is $13$ , which is also the height of $CDP$ . Therefore the area of $CDP$ is $\frac{1}{2}\times13\times84=\boxed{546}$ .

2017 AIME II (Problems • Answer Key • Resources)
Preceded by Problem 9	Followed by Problem 11
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2017 AIME II Problems/Problem 10

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