Proving 2^n < n! using Induction: Where to Start?

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Homework Help Overview

The discussion revolves around proving the inequality \(2^n < n!\) for \(n \geq 4\) using mathematical induction. The original poster has established the base case but is uncertain about the next steps in the inductive process.

Discussion Character

  • Exploratory, Assumption checking, Mathematical reasoning

Approaches and Questions Raised

  • The original poster has confirmed the base case and is seeking guidance on how to proceed with the inductive hypothesis. Another participant suggests writing out the factorial expression and manipulating it to facilitate the proof.

Discussion Status

The discussion is ongoing, with some participants providing insights into the inductive step and others expressing uncertainty about the approach. There is no explicit consensus on the method to be used, but suggestions have been made to explore the relationship between the terms involved in the inequality.

Contextual Notes

The original poster has indicated that the proof must be conducted for \(n \geq 4\) and has already verified the base case for this range. There is a mention of previous assistance received, which may influence the current discussion dynamics.

joshanders_84
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I posted this elsewhere but realized it ought to be in the homework section. I have to use induction to prove that for n>=4, 2^n < n! is true, but I don't know wehre to start. I have the base case proven, but then I don't know where to go after I have my Inductive Hypothesis that it works for all n's greater than 4. Any help would be very appreciated. Thank you
Josh
 
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NateTG helped me in the original posting, so you can disregard this (unless you want to do it for fun!) Thanks
Josh
 
joshanders_84 said:
I posted this elsewhere but realized it ought to be in the homework section. I have to use induction to prove that for n>=4, 2^n < n! is true, but I don't know wehre to start. I have the base case proven, but then I don't know where to go after I have my Inductive Hypothesis that it works for all n's greater than 4. Any help would be very appreciated. Thank you
Josh
Write out the expression for n!

[tex]n! = n(n-1)(n-2)(n-3)...(n-(n-1))[/tex]

Then subtract each term by a positive number so that each term in the product is equal to 2.

AM
 
Induction is the easiest way, I think.

EDIT : Sorry, I just saw the title of your post, and you wanted the induction proof. :smile:

The initial verification for the case of 4 is easy. Say the inequality holds for some [itex]k[/itex].

Then [tex]2^{k+1} = 2.2^k < 2.k! < (k+1).k! = (k+1)![/tex] because [itex](k+1) > 2[/itex] for all [itex]k > 1[/itex].

And you're done.
 
Last edited:

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