1955 AMC 12 Problem 48

Attempt Problem 48 of the 1955 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 1955 AMC 12 solutions, or check the answer key.

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48.

Given triangle ABCABC with medians AE‾,\overline{AE}, BF‾,\overline{BF}, CD‾;\overline{CD}; FH‾\overline{FH} parallel and equal in length to AE‾;\overline{AE}; BH‾\overline{BH} and HE‾\overline{HE} are drawn; FE‾\overline{FE} extended meets BH‾\overline{BH} in G.G. Which one of the following statements is not necessarily correct?
AEHFAEHF is a parallelogram
HE‾=HG‾\overline{HE}=\overline{HG}
BH‾=DC‾\overline{BH}=\overline{DC}
FG‾=34AB‾\overline{FG}=\dfrac34\overline{AB}
FG‾\overline{FG} is a median of triangle BFHBFH
Answer: B
Concepts:mediansvectorsparallelogramaffine geometry
Small hint:
Place A=(0,0),A=(0,0), B=(2,0),B=(2,0), C=(u,v)C=(u,v) and compute the midpoints D,E,FD,E,F
Big hint:
Use FH→=AE→\overrightarrow{FH}=\overrightarrow{AE}, then locate GG where BHBH reaches height v2\frac{v}{2}
Solution:
Set A=(0,0),A=(0,0), B=(2,0),B=(2,0), C=(u,v).C=(u,v). Then D=(1,0),E=(u+22,v2),F=(u2,v2). \begin{aligned} D&=(1,0),\\ E&=\left(\frac{u+2}{2},\frac v2\right),\\ F&=\left(\frac u2,\frac v2\right). \end{aligned} Since FH→=AE→,\overrightarrow{FH}=\overrightarrow{AE}, we get H=(u+1,v).H=(u+1,v). The horizontal line FEFE meets BHBH halfway from BB to H,H, at G=(u+32,v2).G=(\frac{u+3}{2},\frac{v}{2}). These coordinates verify that AEHFAEHF is a parallelogram, BH→=DC→,\overrightarrow{BH}=\overrightarrow{DC}, FG=3AB4,FG=\frac{3AB}{4}, and GG is the midpoint of BH,BH, making FGFG a median. But HE=12u2+v2,HG=12(u−1)2+v2, \begin{aligned} HE&=\frac12\sqrt{u^2+v^2},\\ HG&=\frac12\sqrt{(u-1)^2+v^2}, \end{aligned} which are not generally equal.
Thus, statement B is not necessarily correct.
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