Saturday 20 December 2014

discrete mathematics - How to prove that $4^n>n^2$ using induction...



today I've encountered a question like the following;
$$\text{Prove that }4^n>n^2\text{ using induction.}$$
My Attempts:



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




I have tried to multiply both sides with a $4$ which gave $4^{n+1}>4n^2$ I have tried to turn it out like $4>1^2$ and that gave me $4^{n+1}>n^2\cdot1^2$.....(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 $n\in\mathbb{N}$.



proof





  1. actually $4^1> 1^2$

  2. (I.H.) if $4^n>n^2$ for $n>1$ consider that $4^{n+1}=4\cdot 4^n$. Now use the Inductive Hypothesis (I.H.)



$4\cdot 4^n > 4\cdot n^2 =2^2 \cdot n^2=(2n)^2>(n+1)^2$ as $n>1$



proved, so





  1. For any $n\in \mathbb{N}$ we have $4^n>n^2$



QED
$$
.
$$



To say the truth $4^n > n^{1000}$ for $n>6312$




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


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