1985 AHSME Problems/Problem 3

Problem

In right $\triangle ABC$ with legs $5$ and $12$ , arcs of circles are drawn, one with center $A$ and radius $12$ , the other with center $B$ and radius $5$ . They intersect the hypotenuse at $M$ and $N$ . Then, $MN$ has length:

$[asy] defaultpen(linewidth(0.7)+fontsize(10)); pair A=origin, B=(12,7), C=(12,0), M=12*dir(A--B), N=B+B.y*dir(B--A); real r=degrees(B); draw(A--B--C--cycle^^Arc(A,12,0,r)^^Arc(B,B.y,180+r,270)); pair point=incenter(A,B,C); label("$A$", A, dir(point--A)); label("$B$", B, dir(point--B)); label("$C$", C, dir(point--C)); label("$M$", M, dir(point--M)); label("$N$", N, dir(point--N)); label("$12$", (6,0), S); label("$5$", (12,3.5), E);[/asy]$

$\mathrm{(A)\ } 2 \qquad \mathrm{(B) \ }\frac{13}{5} \qquad \mathrm{(C) \ } 3 \qquad \mathrm{(D) \ } 4 \qquad \mathrm{(E) \ }\frac{24}{5}$

Solution

First of all, from the Pythagorean Theorem, $AB=\sqrt{AC^2+BC^2}=\sqrt{12^2+5^2}=\sqrt{144+25}=\sqrt{169}=13$ . Also, since $AM$ and $AC$ are radii of the same circle, $AM=AC=12$ . Therefore, $MB=AB-AM=13-12=1$ . Also, since $BN$ and $BC$ are radii of the same circle, $BN=BC=5$ . We therefore have $MN=BN-BM=5-1=4, \boxed{\text{D}}$ .

1985 AHSME (Problems • Answer Key • Resources)
Preceded by Problem 2	Followed by Problem 4
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1985 AHSME Problems/Problem 3

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