1950 AMC 12 Problem 49

Attempt Problem 49 of the 1950 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 1950 AMC 12 solutions, or check the answer key.

All problems are used with official legal permission of the Mathematical Association of America (MAA).

49.

A triangle has a fixed base ABAB that is 22 inches long. The median from AA to side BCBC is 1121\dfrac12 inches long and can have any position emanating from A.A. The locus of the vertex CC of the triangle is:
A straight line AB,AB, 1121\dfrac12 inches from AA
A circle with AA as center and radius 22 inches
A circle with AA as center and radius 33 inches
A circle with radius 33 inches and center 44 inches from BB along BABA
An ellipse with AA as focus
Answer: D
Concepts:median (geometry)circletransformation
Small hint:
Let MM be the midpoint of BCBC; then MM moves on a circle centered at AA
Big hint:
In vector form, the midpoint relation gives C⃗=2M⃗−B⃗\vec C=2\vec M-\vec B when AA is the origin
Solution:
Put AA at the origin and regard B,B, C,C, and the midpoint MM of BCBC as vectors. Since AM=32,AM=\tfrac32, the point MM moves on a circle of radius 32\tfrac32 centered at A.A. The midpoint relation gives C=2M−B. C=2M-B. Thus the locus of CC is the image of that circle under a dilation by 22 followed by translation by −B.-B. It is a circle of radius 33 centered at −B.-B.
Because AB=2,AB=2, the point −B-B is 44 inches from BB along the ray BA.BA. Hence the correct answer is D.
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