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Club Challenges -- Triangle Problem [#permalink]
03 Jan 2008, 21:57
--please explain answer
If one of two parallel lines has 7 points on it and the other has 8, how many triangles can be drawn using the points?
* 168 * 196 * 316 * 364 * 455
You can take 2 points out of 8 on one line and combine them with one of the 7 points on the other line, or you can take 2 points from the other line and combine them with one of the 8 points on the original line. Algebraically: 7C_8^2+8C_7^2 = 364 The correct answer is D.