Understanding Factorials and Multiplying by an Integer

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Homework Help Overview

The discussion revolves around understanding factorials, particularly how to handle the factorial of a product involving an integer, such as (2n)!. The original poster expresses confusion about the relationship between factorials and multiplication by an integer, specifically in the context of expanding (2n)!. Participants are exploring the definitions and properties of factorials in this context.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the definition of factorial and its expansion, questioning how (2n)! relates to sequences of integers. There are attempts to clarify the relationship between different representations of factorials and the sequences involved.

Discussion Status

The discussion is active, with participants providing insights and clarifications. Some have offered guidance on understanding the sequences leading to (2n)!, while others are still grappling with the conceptual connections. There appears to be a productive exchange of ideas, with some participants expressing newfound clarity.

Contextual Notes

There is a noted difficulty in visualizing the sequence of integers from n+1 to 2n, which has been a point of confusion for some participants. The original poster's familiarity with factorials is limited, which may be influencing the discussion dynamics.

sandy.bridge
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Homework Statement


Hey all. Not super familiar with using factorials, however, they do pop up occasionally. I understand that n! = 1*2*3*...*n. How do we treat factorial when we are multiplying n by an integer before taking the factorial? I know the answer for expanding (2n)!, however, I do not see why. Thanks in advance.
 
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What don't you see? Think of this:

4! = 4*3*2*1

(2*4)! = 8! = 8*7*6*5*4*3*2*1
 
so (2n)!=(2*1)*(4*3*2*1)*(6*5*4*3*2*1)*...*(2n*(2n-1)*...*1). This can be taken a step further, though correct?
 
sandy.bridge said:
so (2n)!=(2*1)*(4*3*2*1)*(6*5*4*3*2*1)*...*(2n*(2n-1)*...*1). This can be taken a step further, though correct?

I don't think you get it. Look at post #2 again.
 
(2n)!=(2n*(2n-1)*(2n-2)*...*1) ?
 
sandy.bridge said:
(2n)!=(2n*(2n-1)*(2n-2)*...*1) ?
That's more like it!
 
Okay. How exactly does that end up being: 1*2*3*...*n*(n+1)*(n+2)*(n+3)*...*(2n) ?
 
sandy.bridge said:
Okay. How exactly does that end up being: 1*2*3*...*n*(n+1)*(n+2)*(n+3)*...*(2n) ?

How did (2*4)! = 8! end up being 1*2*3* ... *8? What are you not seeing?
 
Look at counting to 2n this way:

1,2,3,4,...,n-2,n-1,n - the sequence of all integers from 1 to n
n+1,n+2,n+3,..., n+n-2,n+n-1,n+n - the sequence of integers from n+1 to 2n
 
  • #10
sandy.bridge said:
(2n)!=(2n*(2n-1)*(2n-2)*...*1) ?
sandy.bridge said:
Okay. How exactly does that end up being: 1*2*3*...*n*(n+1)*(n+2)*(n+3)*...*(2n) ?

You've written it down correctly twice - one of them is in reverse order of the other. How can you not know that they are the same?

1*2 = 2*1 etc
 
  • #11
SteamKing said:
Look at counting to 2n this way:

1,2,3,4,...,n-2,n-1,n - the sequence of all integers from 1 to n
n+1,n+2,n+3,..., n+n-2,n+n-1,n+n - the sequence of integers from n+1 to 2n
For some reason I was having a hard time seeing from n+1 to 2n. I completely see it now. Thanks!
 

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