Math Symbol for CDate: 12/15/97 at 22:50:24 From: Andrew Valovcin Subject: Re: Math symbol for C This is my first time to your site and I find it very interesting and enjoyable. I am puzzled by one symbol of typing math. What does the upper case letter C mean? Like in (2 C 1) (3 C 1) / (47 C 2) = 6/1081. I can't figure it out and before I get any more frustrated I thought I'd better write to you. Thanks for your help. Date: 12/17/97 at 08:12:11 From: Doctor Anthony Subject: Re: Math symbol for C The symbol nCr stands for the number of combinations of r things that can be formed from n different things. If you look in any textbook on probability and statistics you will find that permutations and combinations will feature a good deal in the early chapters. Modern text books now use a different notation for nCr, but it is written on two lines, the n above the r and with brackets around them but no line between. This notation is not suitable for ascii presentation, so we have to revert to the older nCr notation. In ascii we also use C(n,r) to mean the same thing, and this has advantages if we are working with numbers as the numbers themselves stand out more clearly. 10! 10 C 4 = C(10,4) = ------ = 210 4! 6! and if we are dealing with permutations, or arrangements, of 4 things that could be formed from 10 different things, then we write: 10! 10 P 4 = P(10,4) = ----- = 5040 6! The derivation of these expressions is straightforward and you will find them explained in almost any textbook on probability. If you still have difficulty understanding them, write back. -Doctor Anthony, The Math Forum Check out our web site! http://mathforum.org/dr.math/ |
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