Got Homework?
Connect with other students for help. It's a free community.
Here's the question you clicked on:
 0 viewing
probss
Group Title
LIMIT!
using L'Hopital's rules, evaluate
lim x appraoches infinity (cos(x/2))^x^2
 5 months ago
 5 months ago
probss Group Title
LIMIT! using L'Hopital's rules, evaluate lim x appraoches infinity (cos(x/2))^x^2
 5 months ago
 5 months ago

This Question is Open

myininaya Group TitleBest ResponseYou've already chosen the best response.1
Do you know you can write y as e^ln(y) let me know if that helps brb
 5 months ago

myininaya Group TitleBest ResponseYou've already chosen the best response.1
so what I'm say is you can write \[(\cos(x/2))^{x^2}=e^{\ln((\cos(x/2))^{x^2})}=e^{x^2 \ln(\cos(x/2))}=e^{\frac{\ln(\cos(x/2))}{\frac{1}{x^2}}}\]
 5 months ago

probss Group TitleBest ResponseYou've already chosen the best response.0
i think i forgot about it
 5 months ago

myininaya Group TitleBest ResponseYou've already chosen the best response.1
now you did to find this limit \[\lim_{x \rightarrow \infty}\frac{\ln(\cos(x/2))}{\frac{1}{x^2}}\]
 5 months ago

probss Group TitleBest ResponseYou've already chosen the best response.0
wow, seems it is so complicated.
 5 months ago

probss Group TitleBest ResponseYou've already chosen the best response.0
so, we can just take the value of power of exponential? what say you?
 5 months ago

myininaya Group TitleBest ResponseYou've already chosen the best response.1
Instead of writing \[x^2 \ln(\cos(x/2)) \text{ as } \frac{\ln(\cos(x/2))}{\frac{1}{x^2}} \text{ it might be more preferable } \\ \text{ if we write is as } \frac{x^2}{\frac{1}{\ln(\cos(x/2))}}\] still don't know if we should use l'hospital even though it says and that is why I attempted to set the problem up in this way the reason I say that is because the limit as x approaches inf of ln(cos(x/2)) dne since lim x approaches inf of cos(x/2) does not exist since it will oscillate between 1 and 1 but pretending we can use l'hospital since it say use l'hospital i guess we will force the l'hospital
 5 months ago

myininaya Group TitleBest ResponseYou've already chosen the best response.1
@zepdrix the reason i ask is because i only see it used for cases like inf/inf or 0/0
 5 months ago

myininaya Group TitleBest ResponseYou've already chosen the best response.1
not 0/dne or dne/inf
 5 months ago

zepdrix Group TitleBest ResponseYou've already chosen the best response.0
Oh boy D: Good question.. hmm So we have a problem since ln(cosx) is undefined over and over as x>infinity, yah?
 5 months ago

zepdrix Group TitleBest ResponseYou've already chosen the best response.0
Grr I dunno >.< maybe lego man knows
 5 months ago

myininaya Group TitleBest ResponseYou've already chosen the best response.1
maybe sith knows i seen him around a lot doing math like it is nothing
 5 months ago

SithsAndGiggles Group TitleBest ResponseYou've already chosen the best response.0
Wolfram seems to agree; the limit doesn't exist. That's my guess
 5 months ago

sweetburger Group TitleBest ResponseYou've already chosen the best response.0
LHops international house of limits
 5 months ago

KingGeorge Group TitleBest ResponseYou've already chosen the best response.1
Not only does wolfram not say anything, but Mathematica comes up blank as well.
 5 months ago

myininaya Group TitleBest ResponseYou've already chosen the best response.1
Does that mean Mathematica doesn't have the programming to find the limit or that the limit does not exist?
 5 months ago

myininaya Group TitleBest ResponseYou've already chosen the best response.1
i believe the limit does not exist and i also believe we don't need l'hopital (even though it says to use it)
 5 months ago

KingGeorge Group TitleBest ResponseYou've already chosen the best response.1
I think it only tells you that mathematica doesn't have the programming.
 5 months ago

SithsAndGiggles Group TitleBest ResponseYou've already chosen the best response.0
Hmm... power series? With WA, adding more terms to the cosine series makes the limit alternate between infinity and complex infinity... Not sure what that tells us.
 5 months ago

tkhunny Group TitleBest ResponseYou've already chosen the best response.0
Can it converge if it is only intermittently continuous? This is not some countably infinite number of discontinuities (where some Lebesgue measure might provide consistent results). In the logarithm form, there are giant gaps. This is really why introducing the logarithm is a bad idea. It massively modifies the Domain. If you can tell me there is a big number, M, where \(cos(x/2) > 0\;for\;x > M\), then the logarithm is of no concern. There is no such number M.
 5 months ago

KingGeorge Group TitleBest ResponseYou've already chosen the best response.1
This limit can't possibly exist. Since cosine regularly alternates between \(\pm1\), no matter what value of \(x\) we're currently looking at, we can get a larger value such that \(\cos(x/2)\) is 1. Powers have no effect on this, and so you can always get a larger value such that the function outputs 1. But you can also get a larger value whose output will give you 0. So there can't possibly be a limit.
 5 months ago

tkhunny Group TitleBest ResponseYou've already chosen the best response.0
Sequence: \((\cos(4n\pi))^{n^2}\;for\; n\in\mathbb{N}\) It's just ones (1s).
 5 months ago

DominicNg Group TitleBest ResponseYou've already chosen the best response.0
I believe the question is not properly phrase. As x tends infinity, cos(x/2) and sin(x/2) is not defined.
 5 months ago
See more questions >>>
Your question is ready. Sign up for free to start getting answers.
spraguer
(Moderator)
5
→ View Detailed Profile
is replying to Can someone tell me what button the professor is hitting...
23
 Teamwork 19 Teammate
 Problem Solving 19 Hero
 Engagement 19 Mad Hatter
 You have blocked this person.
 ✔ You're a fan Checking fan status...
Thanks for being so helpful in mathematics. If you are getting quality help, make sure you spread the word about OpenStudy.