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We've got the following to prove by induction:

a) [itex]2n+1\le{2^n}[/itex]

b) [itex]n^2\le{2^n}[/itex]

(It is assumed that 0 is a natural number)

a) This inequality is not valid for [itex]n=1,2[/itex], so to prove the inequality one has to show its validity for all [itex]n\ge{3}[/itex]:

1) [itex]A(3):7\le{8}[/itex] (true)

2) Assume that [itex]A(n)[/itex] is true.

[itex]A(n+1): 2(n+1)+1\le{2^{n+1}}[/itex]

[itex]2(n+1)+1=(2n+1)+2\le{2^n+2}\le{2^{n+1}}[/itex], since for [itex]2^n+2\le{2^{n+1}}[/itex] we have [itex]n\ge{1}[/itex]

Is there a difference if I make the induction step this way?:

2)[itex]A(n+1): 2(2n+1)\le{2^{n+1}}[/itex], so to prove A(n) we must show that [itex]2n+3\le{4n+2}[/itex], which is also true for [itex]n\ge{1}[/itex]

The thing that disturbs me is that if we assume 0 to be a natural number the last inequalities in 2) in both cases do not hold...but I think this case doesn't play a role since we are to prove it for n greater 3...does it?

b) This inequality is invalid for all [itex]n=3[/itex], so to show that the inequality is valid we must show that it is valid for all [itex]n\ge{4}[/itex]

1) [itex]A(4)=16\le{16}[/itex] (true)

2) Assume that [itex]A(n)[/itex] is true.

[itex]A(n+1): (n+1)^2\le{2^{n+1}}[/itex]

[itex](n+1)^2=n^2+2n+1\le{2^n+2n+1}\le{2^{n+1}}[/itex]. The last inequality in this string is equivalent to [itex]2n+1\le{2^n}[/itex], which was proved in a).

Here, equally, the same question...Is there a difference if I do it this way?:

2) [itex]A(n+1): 2n^2\le{2^{n+1}}[/itex]

[itex](n+1)^2=n^2+2n+1\le{n^2+3n-3}\le{n^2+n^2-3}\le{2n^2}[/itex]

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# Homework Help: Prove by induction

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