Saturday, 20 December 2014

discrete mathematics - How to prove that 4n>n2 using induction...



today I've encountered a question like the following;
Prove that 4n>n2 using induction.
My Attempts:



I have realised that this works for P(1), my next attempt was p(n)p(n+1)....(1)




I have tried to multiply both sides with a 4 which gave 4n+1>4n2 I have tried to turn it out like 4>12 and that gave me 4n+1>n212.....(2)



After that pointless attempt I've added 2n+1 to both sides but I couldn't figure out still what 2n goes to in the left side...(3)



What are your suggestions?



With the real question being the first one, is there any other way to prove this numerically? (Perhaps in a more entertaining way?:))


Answer



Numerically is not a proof.




Induction works in this way




  1. It is true for n=1

  2. Suppose that is is true for n>1, prove it for n+1

  3. It is true for any nN.



proof





  1. actually 41>12

  2. (I.H.) if 4n>n2 for n>1 consider that 4n+1=44n. Now use the Inductive Hypothesis (I.H.)



44n>4n2=22n2=(2n)2>(n+1)2 as n>1



proved, so





  1. For any nN we have 4n>n2



QED
.



To say the truth 4n>n1000 for n>6312




Indeed induction can start from any n, but this is another story


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