1963 AMC 12 Problem 38

Attempt Problem 38 of the 1963 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 1963 AMC 12 solutions, or check the answer key.

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38.

Point FF is taken on the extension of side ADAD of parallelogram ABCD.ABCD. BFBF intersects diagonal ACAC at EE and side DCDC at G.G. If EF=32EF=32 and GF=24,GF=24, then BEBE equals:

44

88

1010

1212

1616

Answer: E
Concepts:parallelogramvectorratio and proportion
Difficulty rating: 2150
Small Hint:

Parameterize points on FBFB by their fraction of the distance from FF to BB

Big Hint:

If F=tDF=tD in affine coordinates based at A,A, the parameters of GG and EE are t1t\frac{t-1}{t} and tt+1\frac{t}{t+1}

Solution:

Use affine coordinates A=0,A=0, B=u,B=u, D=v,D=v, C=u+v,C=u+v, and F=tv.F=tv. A point on FBFB is su+t(1s)v.su+t(1-s)v. Intersecting DCDC gives sG=t1t,s_G=\frac{t-1}{t}, while intersecting ACAC gives sE=tt+1.s_E=\frac{t}{t+1}. Therefore GFEF=sGsE=t21t2=2432=34, \begin{aligned} \frac{GF}{EF} &=\frac{s_G}{s_E} =\frac{t^2-1}{t^2}\\ &=\frac{24}{32}=\frac34, \end{aligned} so t=2t=2 and sE=23.s_E=\frac{2}{3}. Thus EF=(23)FB=32,EF=(\frac{2}{3})FB=32, making FB=48FB=48 and BE=16.BE=16.

Therefore, the correct answer is E.

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Problem 38 in Other Years

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