If a>-1, show that (1+a)^n>=1+na

  • Thread starter bhoom
  • Start date
In summary: Since k is a positive integer bigger than 1, it is at least 2. Let's check it for k=2(a+1)(1) ≥ 1+a, it holds!I will try to see if it holds for n=k(a+1)^k ≥ 1+ak, and if k=2 a^2+2a+1 ≥ 1+2a, and since a^2 is positive this holds aswell!n=k+1(a+1)*(
  • #1
bhoom
15
0

Homework Statement


[tex]a> -1, (1+a)^n \geq 1+na[/tex]


Homework Equations


[tex]a>-1[/tex]
[tex]a+1> 0[/tex]

The Attempt at a Solution


If I let n=0, and then n=1 i get that 1≥1, and a+1≥ a+1.

Add to each side n+1
[tex](1+a)^{n+1}+(1+a)^{n} \geq 1+na+(1+a(n+1))[/tex]
Then...? Perhaps subtract the original expressions on each side and then evaluate if i can show that n+1-n is ≥ n+1-n
[tex](1+a)^{n+1}-(1+a)^n \geq 1+a(n+1)-(1+an)[/tex]
[tex]a(a+1)^n \geq a[/tex]

However, I don't really know what I'v done. for a>-1, is it obvious that a(a+1)^n≥a?
Or have I attacked the problem in a wrong way? I'm new to induction, and completely green when it comes to bigger than, geq, etc. instead of =
 
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  • #2
bhoom said:

Homework Statement


[tex]a> -1, (1+a)^n \geq 1+na[/tex]


Homework Equations


[tex]a>-1[/tex]
[tex]a+1> 0[/tex]

The Attempt at a Solution


If I let n=0, and then n=1 i get that 1≥1, and a+1≥ a+1.
Either of these serves as your base case. I would use n = 1, though.
bhoom said:
Add to each side n+1
?
bhoom said:
[tex](1+a)^{n+1}+(1+a)^{n} \geq 1+na+(1+a(n+1))[/tex]
Then...? Perhaps subtract the original expressions on each side and then evaluate if i can show that n+1-n is ≥ n+1-n
[tex](1+a)^{n+1}-(1+a)^n \geq 1+a(n+1)-(1+an)[/tex]
[tex]a(a+1)^n \geq a[/tex]

However, I don't really know what I'v done. for a>-1, is it obvious that a(a+1)^n≥a?
Or have I attacked the problem in a wrong way? I'm new to induction, and completely green when it comes to bigger than, geq, etc. instead of =

You started off sort of OK by establishing a base case (in fact, two of them).
There are two more parts in an induction problem.
1) The induction hypothesis, which you assume is true. For your problem, assume that the proposition is true for n = k.
That is, assume that (1 + a)k >= 1 + ka
2) The induction step. Use the induction hypothesis of the previous step to show that the proposition is true for n = k + 1. IOW, show that (1 + a)k + 1 >= 1 + (k + 1)a is true.
 
  • #3
If I set n=k+1 I can write the expression like this[tex](a+1)^k(a+1)\geq 1+(k+1)a[/tex]
[tex](a+1)^k(a+1)\geq 1+ak+a[/tex]
Since we assumed that (1 + a)^k ≥ 1 + ka, can I then look at
[tex](a+1)^k(a+1)-(a+1)^k\geq 1+ak+a-(1+ak)[/tex]
[tex]a(a+1)^k\geq a[/tex]

Since k is at least 0, which gives me a ≥ a.

This whole thing feels very unintuitive, I don't really know when I have proven something.
 
Last edited:
  • #4
bhoom said:
If I set n=k+1 I can write the expression like this


[tex](a+1)^k(a+1)\geq 1+(k+1)a[/tex]
No, you can't do this (above). You have to show this, not start off with it.

Start with (a + 1)k + 1 = (a + 1)*(a + 1)k
Now, what can you do with the (a + 1)k expression?
bhoom said:
[tex](a+1)^k(a+1)\geq 1+ak+a[/tex]
Since we assumed that (1 + a)^k ≥ 1 + ka, can I then look at
[tex](a+1)^k(a+1)-(a+1)^k\geq 1+ak+a-(1+ak)[/tex]
[tex]a(a+1)^k\geq a[/tex]

Since k is at least 0, which gives me a ≥ a.

This whole thing feels very unintuitive, I don't really know when I have proven something.
 
  • #5
In words:

The original problem, if I set up a base(n=0 already tested and proven) n=1[n=n-(n-1)], I get the statement to be true. Then I will try to see what happens if n=k[n=n-(n-k)], 1<k<n.

Then I will get (a+1)^k ≥ 1+ak. If this is true, then k-1 will also be true.
n=k-1[n=n-(n-(k+1))

(a+1)^(k-1) ≥ 1+a(k-1)
=
(a+1)*(a+1)^(k-2) ≥ 1+ak-a
Since k is a positive integer bigger than 1, it is at least 2. Let's check it for k=2
(a+1)(1) ≥ 1+a, it holds!

I will try to see if it holds for n=k

(a+1)^k ≥ 1+ak, and if k=2

a^2+2a+1 ≥ 1+2a, and since a^2 is positive this holds aswell!

n=k+1

(a+1)*(a+1)^k ≥ 1+(k+1)a
=
(a+1)*(a+1)^k ≥ 1+ak+a

If k=2

a^3+3a^2+3a+1 ≥ 1+3a, and since if a is negative it is smaller than -1 and that says that a^3 will be smaller than 3a^2 so again it holds. If my statements are correct, how can they be improved. If not, where did I go wrong?
 
  • #6
a2 ≥ 0

∴ 1 + (k+1)a + a2 ≥ 1 + (k+1)a
 
  • #7
Ahh, I had completely forgotten that I could simplify on both sides of the ≥. Thanks both of you.
 
  • #8
bhoom said:
In words:

The original problem, if I set up a base(n=0 already tested and proven) n=1[n=n-(n-1)], I get the statement to be true. Then I will try to see what happens if n=k[n=n-(n-k)], 1<k<n.
Why do you have the stuff in the brackets? n=1[n=n-(n-1)] and n=k[n=n-(n-k)]


bhoom said:
Then I will get (a+1)^k ≥ 1+ak. If this is true, then k-1 will also be true.
n=k-1[n=n-(n-(k+1))

(a+1)^(k-1) ≥ 1+a(k-1)
=
(a+1)*(a+1)^(k-2) ≥ 1+ak-a
Since k is a positive integer bigger than 1, it is at least 2. Let's check it for k=2
(a+1)(1) ≥ 1+a, it holds!

I will try to see if it holds for n=k

(a+1)^k ≥ 1+ak, and if k=2

a^2+2a+1 ≥ 1+2a, and since a^2 is positive this holds aswell!

n=k+1

(a+1)*(a+1)^k ≥ 1+(k+1)a
=
(a+1)*(a+1)^k ≥ 1+ak+a
[/quote]No, this is not right. You are assuming what you need to prove.

Also, two inequalities are not equal, so don't connect them with =.
bhoom said:
If k=2

a^3+3a^2+3a+1 ≥ 1+3a, and since if a is negative it is smaller than -1 and that says that a^3 will be smaller than 3a^2 so again it holds.


If my statements are correct, how can they be improved. If not, where did I go wrong?

It's hard to follow what you are doing, but it looks to me like you are just establishing some more base cases.

There's a certain template that you should follow.
1) Establish that the proposition is true for some base case (such as n = 1). You have done that.
2) Assume that the proposition is true for n = k.
IOW, assume that (1 + a)k >= 1 + ka.
3) Using the assumption in #2, show that the proposition is true for n = k + 1.
IOW, show that (1 + a)k + 1 >= 1 + (k + 1)a
Take a look at what I did in post #4.
 
  • #9
Ok, I'm off to sleep now but I will do another take on it tomorrow. The brackets were to show that 0<1<k<n<n+1.
Thanks so far though.
 
  • #10
bhoom said:
Ok, I'm off to sleep now but I will do another take on it tomorrow. The brackets were to show that 0<1<k<n<n+1.
You don't need that.
 
  • #11
Ok, I will go from my base case, n=1 is true.

Now I assume n=k is true. From that assumption I show that n=k+1 is true.

(a+1)^(k+1) ≥ 1+a(k+1)

(a+1)*(a+1)^k ≥ 1+ak+a

(a+1) ≥ (1+ak+a)/(a+1)^k

(a+1) ≥ (1+ak+a)(a+1)^-k

or perhaps

(a+1)^(k+1) ≥ 1+a(k+1)

(a+1)*(a+1)^k ≥ 1+ak+a

-ak-a+(a+1)*(a+1)^k ≥ 1

Are my induction-process-assumptions correct? So I know that I can focus on algebraic operations on my "(a+1)^(k+1) ≥ 1+a(k+1)" expression to show that it is true for n=k+1.
 
  • #12
bhoom said:
Ok, I will go from my base case, n=1 is true.

Now I assume n=k is true. From that assumption I show that n=k+1 is true.
Don't say that n = 1 is true and that n = k is true. Instead, say that that the proposition is true for n = 1, and that the proposition is true for n = k. Your job is to show that the proposition is true for n = k + 1.
bhoom said:
(a+1)^(k+1) ≥ 1+a(k+1)
When you write the above, you are tacitly assuming that it is true when you apply operations to both sides. You can't do that - you need to provide the work that shows it is true.

Your work should look something like this:
(a + 1)k+1 = (a + 1)*(a + 1)k = ...<stuff>... ≥ ...<other stuff>... ≥ 1 + a(k + 1)

In that way, you will have shown that (a + 1)k+1 ≥ 1 + a(k + 1). You need to fill in where I have <stuff> and <other stuff>.

Something you need to fill in is what you can exchange for (a + 1)k, based on your induction hypothesis.

bhoom said:
(a+1)*(a+1)^k ≥ 1+ak+a

(a+1) ≥ (1+ak+a)/(a+1)^k

(a+1) ≥ (1+ak+a)(a+1)^-k

or perhaps

(a+1)^(k+1) ≥ 1+a(k+1)

(a+1)*(a+1)^k ≥ 1+ak+a

-ak-a+(a+1)*(a+1)^k ≥ 1

Are my induction-process-assumptions correct? So I know that I can focus on algebraic operations on my "(a+1)^(k+1) ≥ 1+a(k+1)" expression to show that it is true for n=k+1.
 
  • #13
I think i got it earlier today. I am the phone atm, so I can't supply any details, but I ended up comparing (1+ak)(a+1) \geq (1+ka+a)
 

1. How do you prove that (1+a)^n >= 1+na when a>-1?

To prove this inequality, we can use mathematical induction. First, we will show that the statement is true for n=1. (1+a)^1 = 1+a >= 1+a, which is true. Next, we assume that the inequality holds for some arbitrary natural number k. (1+a)^k >= 1+ka. Then, we will prove that the inequality also holds for k+1. (1+a)^(k+1) = (1+a)^k * (1+a) >= (1+ka) * (1+a) = 1+(k+1)a+ka^2 >= 1+(k+1)a. Since a>-1, ka^2 >= 0, therefore the inequality holds for k+1. By mathematical induction, the inequality holds for all natural numbers n.

2. Why is a>-1 a necessary condition for the inequality to hold?

This inequality only holds when a>-1 because if a<=-1, then (1+a)^n < 1+na. For example, if a=-2 and n=2, (1+a)^2 = (-1)^2 = 1, but 1+2(-2) = -3. Therefore, a>-1 is necessary for the inequality to hold.

3. Can this inequality be extended to negative values of n?

No, this inequality only holds for natural numbers n. If n is a negative integer, the inequality will not hold. For example, if a=0.5 and n=-1, (1+a)^(-1) = 2, but 1+(-1)(0.5) = 0.5. Therefore, this inequality cannot be extended to negative values of n.

4. Is this inequality reversible?

Yes, this inequality is reversible. If we have (1+a)^n >= 1+na, we can divide both sides by (1+a) to get (1+a)^(n-1) >= 1+(n-1)a. This shows that the inequality also holds for (n-1), and we can continue this process until we reach n=1, which we proved in the first question. Therefore, this inequality is reversible.

5. Can this inequality be used in other mathematical proofs?

Yes, this inequality can be used in various mathematical proofs, particularly in proving other inequalities or in induction proofs. It can also be used in calculus and analysis to simplify calculations and solve problems.

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