Thread: Terrible idea
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Old September 18th, 2015, 08:07 AM
Doug Kerr Doug Kerr is offline
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Join Date: May 2006
Location: Alamogordo, New Mexico, USA
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Hi, Fahim,

Originally Posted by fahim mohammed View Post

As to your answer to a 'teacher's' question as to the number of ways 2 objects can be arranged in a group of 3....being zero ( of course ' zero ' long predates either of us and has somehow managed to survive thus far )...I would say there is an ' x and y ' and a ' z '. The ' z ' being a joker.
We are all teachers. But I believe that question came from a joker.

In any case, it is said that, using the broader meaning of "permutation", with variables n and r, that is, "n things taken r at a time":
P(n,r) = 0 for r>n [The case for the recent question]

P(n,r) = n!/(n-r)! otherwise
We must take 0! to be 1, which makes this work in the case of a true permutation (where r = n).
This value, P(n,r) is formally described as the number of r-permutations of n.

Best regards,

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