# 1964 AHSME Problems/Problem 29

## Problem

In this figure $\angle RFS=\angle FDR$, $FD=4$ inches, $DR=6$ inches, $FR=5$ inches, $FS=7\dfrac{1}{2}$ inches. The length of $RS$, in inches, is:

$\textbf{(A) }\text{undetermined}\qquad\textbf{(B) }4\qquad\textbf{(C) }5\dfrac{1}{2}\qquad\textbf{(D) }6\qquad\textbf{(E) }6\dfrac{1}{2}\qquad$

$[asy] import olympiad; draw((0,0)--7.5*dir(0)); draw((0,0)--5*dir(55)); draw((0,0)--4*dir(140)); draw(4*dir(140)--5*dir(55)--7.5*dir(0)); markscalefactor=0.1; draw(anglemark((0,0),4*dir(140),5*dir(55))); draw(anglemark(7.5*dir(0),(0,0),5*dir(55))); label("F",(0,0),dir(240)); label("D",4*dir(140),dir(180)); label("R",5*dir(55),dir(80)); label("S",7.5*dir(0),dir(-25)); label("4",2*dir(140),dir(230)); label("5",2.5*dir(55),dir(155)); label("6",(4*dir(140)+5*dir(55))/2,dir(100)); label("7\frac{1}{2}",3.75,dir(-90)); [/asy]$

## Solution

We examine $\triangle RDF$ and $\triangle SFR$. We are given $\angle RDF \cong \angle SFR$. Also note that $\frac{SF}{RD} = \frac{7.5}{6} = 1.25$ and $\frac{FR}{DF} = \frac{5}{4} = 1.25$, so $\frac{SF}{RD} = \frac{FR}{DF}$.

If two triangles have two pairs of sides that are proportional, and the included angles are congruent, then the two triangles are similar by SAS congruence. Therefore, the third pair of sides must also be in the same proportion, so

$\frac{SF}{RD} = \frac{FR}{DF} = \frac{RS}{FR} = 1.25$

$\frac{RS}{FR} = 1.25$

$\frac{RS}{5} = 1.25$

$RS = 6.25$, which is answer $\boxed{\textbf{(E) }}$

## Solution 2

Let $\angle RFS = \angle FDR = \theta$. By Cosine Law, we have: \begin{align*} RF^2 &= DR^2 + DF^2 - 2(DR)(DF)cos(\theta) \\ 5^2 &= 6^2 + 4^2 - 2(6)(4)cos(\theta) \\ cos(\theta) &= \frac{6^2+4^2-5^2}{2(6)(4)} = \frac{9}{16} \end{align*} Applying Cosine Law again in $\triangle RFS$, we have: \begin{align*} RS^2 &= RF^2 + FS^2 - 2(RF)(FS)cos(\theta) \\ RS^2 &= 25 + (\frac{15}{2})^2 - 2(5)(\frac{15}{2})(\frac{9}{16}) \\ RS^2 &= \frac{625}{16} \end{align*} giving us $RS = \frac{25}{4}$, which is the answer $\boxed{\textbf{(E)}}$. -nullptr07