osanseviero
  • osanseviero
If I have 5 letters (A,B,C,D,E), and I want to form groups of three letters. a) How many can I form? (AAA, AAB, AAC, etc) b) How many without repeating? (ABC, ABD, etc)
Mathematics
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osanseviero
  • osanseviero
If I have 5 letters (A,B,C,D,E), and I want to form groups of three letters. a) How many can I form? (AAA, AAB, AAC, etc) b) How many without repeating? (ABC, ABD, etc)
Mathematics
chestercat
  • chestercat
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anonymous
  • anonymous
a) thats a combination of 3 from 5..or 5 C 3
osanseviero
  • osanseviero
So 125 in the first one, and 20 in the second one. Why is it n(n-1)?
osanseviero
  • osanseviero
:/

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Hero
  • Hero
If I'm not mistaken the first one is a permutation since elements are allowed to repeat. Which means the total number of three letter formations should be \(n^r=5^3 = 125\) The second question is a combination since elements are not allowed to repeat. So I imagine that it is \(5C3\)
osanseviero
  • osanseviero
Ok, let me check :)
osanseviero
  • osanseviero
I searched an in a permutation elements cant be repeated :/
Hero
  • Hero
You should do more research on permutations.
osanseviero
  • osanseviero
So the basic formulas are \[n ^{r}\] if they can be repeated and NCr, when they cant be repeated?
Hero
  • Hero
This resource might help clear things up a bit more: http://www.mathsisfun.com/combinatorics/combinations-permutations.html
osanseviero
  • osanseviero
Thanks :)
Hero
  • Hero
yw
osanseviero
  • osanseviero
Yep, this helped me a lot :)
osanseviero
  • osanseviero
I am seeing that the formula of the combinations without repetition and permutation without repetition are the same. How can I know if this if a permutation or a combination? (the order matters here?)
Hero
  • Hero
They're not the same :)
osanseviero
  • osanseviero
oops, not the same, I reduce by r one more time. But how can I know in this case if it is permutation or combination?
osanseviero
  • osanseviero
(from the question?, how can I know if order matters?
osanseviero
  • osanseviero
So it is 125 and 10 :)

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