Difference between revisions of "2009 AMC 8 Problems/Problem 20"
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Revision as of 00:49, 5 July 2013
Problem
How many non-congruent triangles have vertices at three of the eight points in the array shown below?
![[asy]dot((0,0)); dot((0,.5)); dot((.5,0)); dot((.5,.5)); dot((1,0)); dot((1,.5)); dot((1.5,0)); dot((1.5,.5));[/asy]](http://latex.artofproblemsolving.com/f/0/c/f0cff29537e2aff0fa79e3cca763dcb4be379f9c.png)
Solution
Assume the base of the triangle is on the bottom four points, because a congruent triangle can be made by reflecting the base on the top four points. For a triangle with a base of length 
, there are 
triangles. For a triangle with a base of length 
, there are triangles. For length , there are . In total, the number of non-congruent triangles is 
.
See Also
| 2009 AMC 8 (Problems • Answer Key • Resources) | ||
| Preceded by Problem 19 |
Followed by Problem 21 | |
| 1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
| All AJHSME/AMC 8 Problems and Solutions | ||
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions. 
