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yosib_2
Newbie
Joined: 11 Nov 2004
Posts: 2
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 Posted: 11 Nov 2004 11:12:12 am Post subject: |
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hi there
this is my first time here so hi everybody .
this is the first time i have a problem calculating an integral with the ti89 calculator.
the problem :
integrate the function f(x)= 1/ ( cosh(x) )^2
the answer as i have checked in the integral table and with maple 9.5 software is
tanh(x)
but the answer in the ti calculator is : -2/( exp(2*x)+1 )
(i used delvar so x is a parameter)
please wating for an aswer
thank everybody
yosi |
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yosib_2
Newbie
Joined: 11 Nov 2004
Posts: 2
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| Posted: 11 Nov 2004 11:12:49 am Post subject: |
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hi there
this is my first time here so hi everybody .
this is the first time i have a problem calculating an integral with the ti89 calculator.
the problem :
integrate the function f(x)= 1/ ( cosh(x) )^2
the answer as i have checked in the integral table and with maple 9.5 software is
tanh(x)
but the answer in the ti calculator is : -2/( exp(2*x)+1 )
(i used delvar so x is a parameter)
please wating for an aswer
thank everybody
yosi |
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shadowing
Powered by 64
Calc Guru
Joined: 06 Jan 2004
Posts: 1002
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| Posted: 11 Nov 2004 11:14:32 am Post subject: |
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Umm... Hi there yoshib_2! I don't know what are integrals. If you own a 89T, i could help. Can you specify about your calculator? Can you also explain whats an integral?
Srry, but I'm only an eight grader.
EDIT: OK, I found out whats an integral.
Last edited by Guest on 12 Nov 2004 11:39:32 am; edited 1 time in total |
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DarkerLine
ceci n'est pas une |
Super Elite (Last Title)
Joined: 04 Nov 2003
Posts: 8328
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| Posted: 11 Nov 2004 12:12:28 pm Post subject: |
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Great help there shadowing.
Technically, there is no problem. If you put expand(tanh(x)) into your TI-89, you will find that tanh(x) = 1 - 2/( exp(2*x)+1 ). Therefore the two answers are equivalent except by a constant difference of 1 (which doesn't matter when calculating integrals). Unfortunately, this usually isn't what you want.
The TI-89 does have problems expressing answers in terms of sinh, cosh, and tanh (it always wants to express them in terms of e^x). There is no good way to get around this. This is the best you can do:
If you know that the answer should be in terms of, for example, tanh(x), you can say
<answer> | x = tanh^-1(y)
You will then get something in terms of y. If you MANUALLY substitute tanh(x) for y you will get the right answer. |
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Brazucs
I have no idea what my avatar is.
Super Elite (Last Title)
Joined: 31 Mar 2004
Posts: 3349
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| Posted: 11 Nov 2004 04:33:46 pm Post subject: |
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Oh yes... I've noticed some of that too.
You see, for some reason, sometimes the 89 miscalculates some derivatives and integrals, especially when they are extremely complex (and 2(especially) when they involve trig functions like cosh())
It must have been an error in their codebecause even when you plug in a number for -2/( exp(2*x)+1 ) and tanh(x), it's not the same. |
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Jeremiah Walgren
General Operations Director
Know-It-All
Joined: 24 May 2003
Posts: 1937
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| Posted: 11 Nov 2004 06:14:51 pm Post subject: |
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| Something worth looking into - a Bug List maintained by Bhuvanesh Bhatt. It's for the 68k family. |
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DarkerLine
ceci n'est pas une |
Super Elite (Last Title)
Joined: 04 Nov 2003
Posts: 8328
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| Posted: 11 Nov 2004 06:17:01 pm Post subject: |
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| this has been answered in the other topic, someone please delete this one. |
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Jeremiah Walgren
General Operations Director
Know-It-All
Joined: 24 May 2003
Posts: 1937
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| Posted: 11 Nov 2004 06:18:29 pm Post subject: |
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shadowing wrote: Umm... Hi there yoshib_2! I don't know what are integrals. If you own a 89T, i could help. Can you specify about your calculator? Can you also explain whats an integral?
Srry, but I'm only an eight grader.
Why are you replying if you don't even know anything relating to the problem?
By the way, is it just me or was this topic posted twice?
Edit: In case one of the topics is deleted.
Something worth looking into - a Bug List maintained by Bhuvanesh Bhatt. It's for the 68k family.
Last edited by Guest on 11 Nov 2004 06:20:23 pm; edited 1 time in total |
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Brazucs
I have no idea what my avatar is.
Super Elite (Last Title)
Joined: 31 Mar 2004
Posts: 3349
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| Posted: 11 Nov 2004 06:24:53 pm Post subject: |
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Sir Robin wrote: Technically, there is no problem.
It's not just you, there ARE two topics... they should be merged. He's just replying for postcount.
And by the way, there IS a problem because -2/(exp(2*x)+1) does not equal tanh(x). I might be wrong but try plugin in a number...
Quote: Definite Integrate returns incorrect results for some divergent integrals
There you go. Maybe they should add that one in.
Last edited by Guest on 12 Nov 2004 02:39:47 am; edited 1 time in total |
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Jeremiah Walgren
General Operations Director
Know-It-All
Joined: 24 May 2003
Posts: 1937
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| Posted: 11 Nov 2004 06:37:12 pm Post subject: |
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brazucs16 wrote: He's just replying for postcount.
I gave up on that a looong time ago.
Though, now that you bring it up, I probably don't have a reason for posting here anymore...
Something else to look at. |
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AlienCC
Creative Receptacle!
Know-It-All
Joined: 24 May 2003
Posts: 1927
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| Posted: 11 Nov 2004 09:53:29 pm Post subject: |
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Topics are now merged, please try to be more careful in not submitting the same topic twice in the future.
--AlienCC |
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DarkerLine
ceci n'est pas une |
Super Elite (Last Title)
Joined: 04 Nov 2003
Posts: 8328
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| Posted: 12 Nov 2004 06:03:14 pm Post subject: |
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Quote: 2/(exp(2*x)+1) does not equal tanh(x)
True, but they are off by a constant value of 1, regardless of x. Which is irrelevant when computing indefinite integrals because there's a constant term anyway. |
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