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
- It is true for $n=1$
- Suppose that is is true for $n>1$, prove it for $n+1$
- It is true for any $n\in\mathbb{N}$.
proof
- actually $4^1> 1^2$
- (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
- 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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