1964 AMC 12 Problem 29

Attempt Problem 29 of the 1964 AMC 12 below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1964 AMC 12 solutions, or check the answer key.

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29.

In this figure ∠RFS=∠FDR,\angle RFS=\angle FDR, FD=4FD=4 inches, DR=6DR=6 inches, FR=5FR=5 inches, and FS=712FS=7\dfrac12 inches. The length of RS,RS, in inches, is:
undetermined
44
5125\dfrac12
66
6146\dfrac14
Answer: E
Concepts:similarityangle chasingratio and proportion
Small hint:
Compare triangles RFSRFS and FDRFDR around the marked equal angles
Big hint:
The adjacent side ratios are 54=1512\frac{5}{4}=\frac{15}{12}
Solution:
At the equal included angles, FRFD=54,FSDR=1512=54. \begin{gathered} \frac{FR}{FD}=\frac54,\\ \frac{FS}{DR}=\frac{15}{12}=\frac54. \end{gathered} Thus △RFS∼△FDR\triangle RFS\sim\triangle FDR by SAS. Side RSRS corresponds to FR=5,FR=5, so RS=(54)⋅5=254=614.RS=(\frac{5}{4})\cdot5=\frac{25}{4}=6\frac14.
Therefore, the correct answer is E.
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