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how do you prove 1+ 1/2 + 1/3 + .... + 1/1000 < 10 ?

how do you prove 1+ 1/2 + 1/3 + .... + 1/1000 < 10 ?

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Reply 1
BowToTheFlyingCircus
how do you prove 1+ 1/2 + 1/3 + .... + 1/1000 < 10 ?


Count them on your fingers - (Have you heard of the harmonic series? I suggeest you look this up on google).
Sum of a series? Look in your formula book :biggrin:
Reply 3
There are many methods. My personal favourite is to approximate it by an integral.
Reply 4
SimonM
There are many methods. My personal favourite is to approximate it by an integral.


:yep: Probably easiest method
Reply 5
1 + 1/2 + 1/3 + 1/4 + 1/5 .... < 1 + 1/2 + 1/2 + 1/4 + 1/4 + 1/4 + 1/4 ..... < 1 + [1/2 + 1/2] + [1/4 + .... < 10
Reply 6
Oops, I misread the question the first time.

Try to rewrite as the sum of lots of sums to 1. If you can show that this sequence is less than 10 of these sums, you're done.

Spoiler

Reply 7
n1r4v
1 + 1/2 + 1/3 + 1/4 + 1/5 .... < 1 + 1/2 + 1/2 + 1/4 + 1/4 + 1/4 + 1/4 ..... < 1 + [1/2 + 1/2] + [1/4 + .... < 10


Really? 1/3 is smaller than 1/2
Reply 8
...
Reply 9
Smiling...Rain
Sum of a series? Look in your formula book :biggrin:


I doubt this series will be in the formula book?

Unless your booklet had Euler's constant?

EDIT: I think Simon's suggestion is the easest way.
Reply 10
DeanK2
I doubt this series will be in the formula book?

Unless your booklet had Euler's constant?


Even if it does. There is no closed form for the sum of the harmonic series (as far as I'm aware)
Reply 11
SimonM
Really? 1/3 is smaller than 1/2


Do you mind explaining what you're trying to imply?
Reply 12
SimonM
Really? 1/3 is smaller than 1/2


?
:smile:
Reply 13
n1r4v
Do you mind explaining what you're trying to imply?


No idea, was on another planet
Reply 14
Agrippa
?
:smile:


???
Reply 15
DeanK2
???


???
:p:

Spoiler

Your series is less than:

1 + 1/2 + 1/2 + 1/4 + 1/4 + 1/4 + 1/4 + 1/8 + 1/8 + ...

Which is 1 + 2(1/2) + 4(1/4) + 8(1/8) + ... + 512(1/512)

Which is 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1

Which is 10

So your series is less than 10


I haven't explained it in the best possible way, but you see what I mean...
n1r4v
1 + 1/2 + 1/3 + 1/4 + 1/5 .... < 1 + 1/2 + 1/2 + 1/4 + 1/4 + 1/4 + 1/4 ..... < 1 + [1/2 + 1/2] + [1/4 + .... < 10


Don't understand why you can do that? and how do you then know if its less than 10?
Reply 18
The integral method (by comparing areas)

110011xdx<n=110001n<1+110001xdx\displaystyle \int_1^{1001} \frac{1}{x} \, dx < \sum_{n=1}^{1000} \frac{1}{n} < 1+\int_1^{1000} \frac{1}{x} \, dx

Which shows that it is 6.90875...<n=110001n<7.907755...\displaystyle6.90875... < \sum_{n=1}^{1000} \frac{1}{n} < 7.907755...

I expect I've made an OBO

Also

n=110001n7.4854708605503449126565182043339001765216791697088\displaystyle \sum_{n=1}^{1000} \frac{1}{n} \approx 7.4854708605503449126565182043339001765216791697088
Reply 19
tazarooni89
Your series is less than:

1 + 1/2 + 1/2 + 1/4 + 1/4 + 1/4 + 1/4 + 1/8 + 1/8 + ...

Which is 1 + 2(1/2) + 4(1/4) + 8(1/8) + ... + 512(1/512)

Which is 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1

Which is 10

So your series is less than 10


I haven't explained it in the best possible way, but you see what I mean...


I've seen this method before, and (I think) I follow it, but how do you know when/how to do that?

Like, where do you pull the 1/8 from? Obviously it's 2^3, but why is it significant?

If I got this question in a test/interview, I'd never think to do that.

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