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Sequences

I've tried uploading the photo of the question directly, but it's not working.
The question is:
http://i60.tinypic.com/2mha99l.jpg

I'm struggling to show it's decreasing. I know a sequence is decreasing if (for all n):
xn+1xn0x_{n+1}-x_n\leq 0 or xn+1xn1\frac{x_{n+1}}{x_n}\leq 1 i.e.
xn(1exn)exn0\frac{x_n(1-e^{x_n})}{e^{x_n}}\leq 0 or xnexnxn=1exn1\frac{x_n}{e^{x_n}x_n}=\frac{1}{e^{x_n}}\leq 1

In an example in my notes, we used proof by induction, but I can't work out what the hypothesis should be.
xn>0x_n>0

exn>1e^{x_n}>1
Reply 2
Original post by Indeterminate
Well, it can only be decreasing if 1 is the largest number in the sequence. So you need to prove that, given x1=1x_1 = 1

xn+1=xnexn1nNx_{n+1} = \frac{x_n}{e^{x_n}} \leq 1 \forall n \in \mathbb{N}

which is equivalent to the statement that

xnexnnNx_n \leq e^{x_n} \forall n \in \mathbb{N}

and there's your hypothesis :smile:


Sorry but I don't see how this proves it is decreasing? I see it proves that all terms are less than or equal to 1. :confused:


Original post by BuryMathsTutor
xn>0x_n>0

exn>1e^{x_n}>1


Hmm, so possibly:
Hypothesis is: exn>1e^{x_n}>1
True for n=1 since e>1

Then assuming it's true for n,
xnxnexn=xn+1x_n\geq \frac{x_n}{e^{x_n}}=x_{n+1}
Which shows it's decreasing. :biggrin:


It seems so simple now! Thanks.
Original post by rayquaza17
Sorry but I don't see how this proves it is decreasing? I see it proves that all terms are less than or equal to 1. :confused:




Hmm, so possibly:
Hypothesis is: exn>1e^{x_n}>1
True for n=1 since e>1

Then assuming it's true for n,
xnxnexn=xn+1x_n\geq \frac{x_n}{e^{x_n}}=x_{n+1}
Which shows it's decreasing. :biggrin:


It seems so simple now! Thanks.


Sorry, yes, of course you're right. I realised & deleted my post but it seems you hit the quote button before that :tongue:
Original post by rayquaza17



It seems so simple now! Thanks.


No problem Rayquaza, us dragon's got to look out for each other :colonhash:
Reply 5
Original post by Loafing.Charizard
No problem Rayquaza, us dragon's got to look out for each other


Charizard isn't a dragon mate (unless he holds Charizardite X)
You're just pretending to be a Pokemon.
Original post by rayquaza17
Charizard isn't a dragon mate (unless he holds Charizardite X)You're just pretending to be a Pokemon.


Exactly. So I am a dragon now :rolleyes:

Lmao you know more about Pokémon than me : o. Do you play/watch it still or nah?
Reply 7
Original post by Loafing.Charizard
Exactly. So I am a dragon now :rolleyes:

Lmao you know more about Pokémon than me : o. Do you play/watch it still or nah?


Nah, I mostly play on my xbox nowadays. I don't get much spare time though tbh. :frown:

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