Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Factorial in example problem, I'm confused

  1. Dec 1, 2004 #1
    I'm confused from one step to the next in the book.

    In the first step, they have (n+1)!

    then in the next step the write that as (n+1)n!


    so (n+1)! = (n+1)n! ?

    I don't understand that, can someone explain it to me ? ( I am very unfamiliar with factorials, this is like some of the first times I have seen it in computations.
     
  2. jcsd
  3. Dec 1, 2004 #2
    Let's take a general case:
    [tex]a!=a(a-1)(a-2)(a-3)...[/tex]
    So in the problem:
    [tex](n+1)!=(n+1)(n+1-1)(n+1-2)(n+1-3)(n+1-4)...=(n+1)(n)(n-1)(n-2)(n-3)...[/tex]
    Now you can see that the terms after [itex](n+1)[/itex] are just [itex]n![/itex], according to our general definition above, so:
    [tex](n+1)!=(n+1)n![/tex]
     
  4. Dec 1, 2004 #3
    The easiest way to see it is a concrete example I think. Take n as 5... then (n+1)! is 6! , which is 6*5*4*3*2*1. (n+1)n! is the same thing because n+1 is 6, then n! is 5*4*3*2*1, giving you 6*5*4*3*2*1.

    edit: posted late I guess :rolleyes:
     
  5. Dec 1, 2004 #4
    n! = 1 *2 * 3 ... *n

    (n + 1)! = 1 * 2 * 3 ... *n *(n+1)

    Since 1*2*3...*n = n!, you can rewrite the statement so that

    (n+1)! = (n+1)n!
     
  6. Dec 1, 2004 #5
    Ohhh i get it now, thanks guys! all of your responses helped me visualize it, ty.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook