1959 AMC 12 Problem 26

Attempt Problem 26 of the 1959 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 1959 AMC 12 solutions, or check the answer key.

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

26.

The base of an isosceles triangle is 2.\sqrt2. The medians to the legs intersect each other at right angles. The area of the triangle is:
1.51.5
22
2.52.5
3.53.5
44
Answer: A
Concepts:isosceles trianglemedian (geometry)coordinate geometryarea
Small hint:
Place the base endpoints symmetrically about the origin and the third vertex on the perpendicular bisector
Big hint:
Write direction vectors for the two medians and set their dot product equal to zero
Solution:
Put the base endpoints at A=(−22,0)A=(-\frac{\sqrt2}{2},0) and B=(22,0),B=(\frac{\sqrt2}{2},0), and let the third vertex be C=(0,h).C=(0,h). The median from AA has direction (324,h2), \left(\frac{3\sqrt2}{4},\frac h2\right), while the median from BB has direction (−324,h2). \left(-\frac{3\sqrt2}{4},\frac h2\right). Their dot product is zero, so −98+h24=0, -\frac98+\frac{h^2}{4}=0, and h=322.h=\frac{3\sqrt2}{2}. Thus the area is 12⋅2⋅322=32. \frac12\cdot\sqrt2\cdot\frac{3\sqrt2}{2}=\frac32.
Therefore, the correct answer is A.
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