Posted by ana on Sunday, March 7, 2010 at 7:38pm.
At Maryland Middle School, there are 9 mathematics classes. If two mathematics classes are selected for a school competition, what is the number of possible outcomes?

math  Damon, Sunday, March 7, 2010 at 7:49pm
combinations of 2 out of 9
C(n,r) = n!/[(nr)! (r!) ]
= 9! /[ 7! * 2! ]
= 9*8 *7! / [ 7! * 2]
=9*4
= 36

math  Damon, Sunday, March 7, 2010 at 7:53pm
Or use Pascal's triangle
row 9 is
1 9 36 84 126 126 84 36 9 1
so r = 0 > 1
r = 1 = 9 of course
r = 2 = 36
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