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Thread Subject:
<<<<root>>>>help ME Plz

Subject: <<<<root>>>>help ME Plz

From: tavirapak

Date: 9 Sep, 2010 09:19:22

Message: 1 of 19

An oscillating current in an electric is described by

I = 10*(exp(-t))*sin(2*pi*t)

Where t is in seconds. Determine all values of t such that.

Subject: <<<<root>>>>help ME Plz

From: Roswa Nito

Date: 9 Sep, 2010 09:58:04

Message: 2 of 19

"tavirapak " <belove_ng@hotmail.com> wrote in message <i6a8qq$ohb$1@fred.mathworks.com>...
> An oscillating current in an electric is described by
>
> I = 10*(exp(-t))*sin(2*pi*t)
>
> Where t is in seconds. Determine all values of t such that.


Hi Tavirapak!!

Can you please complete the sentence?? Thank you!!

Subject: <<<<root>>>>help ME Plz

From: tavirapak

Date: 9 Sep, 2010 16:13:04

Message: 3 of 19

Sorry...I foget So...
> > I = 10*(exp(-t))*sin(2*pi*t)
> >
> > Where t is in seconds. Determine all values of t such that. I=2

THANK YOU VERRY MUCH

Subject: <<<<root>>>>help ME Plz

From: Steven_Lord

Date: 9 Sep, 2010 17:07:09

Message: 4 of 19



"tavirapak " <belove_ng@hotmail.com> wrote in message
news:i6b12g$374$1@fred.mathworks.com...
> Sorry...I foget So...
>> > I = 10*(exp(-t))*sin(2*pi*t)
>> >
>> > Where t is in seconds. Determine all values of t such that. I=2
>
> THANK YOU VERRY MUCH

Look at FZERO.

--
Steve Lord
slord@mathworks.com
comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ
To contact Technical Support use the Contact Us link on
http://www.mathworks.com

Subject: <<<<root>>>>help ME Plz

From: tavirapak

Date: 9 Sep, 2010 17:46:09

Message: 5 of 19

<<THANK YOU FOR HELP>>

Subject: <<<<root>>>>help ME Plz

From: Walter Roberson

Date: 9 Sep, 2010 19:27:27

Message: 6 of 19

On 10-09-09 11:13 AM, tavirapak wrote:
> Sorry...I foget So...
>> > I = 10*(exp(-t))*sin(2*pi*t)
>> > > Where t is in seconds. Determine all values of t such that. I=2

There are an infinite number of solutions. Consider what happens when t is
just slightly different than a negative integer.

Subject: <<<<root>>>>help ME Plz

From: tavirapak

Date: 10 Sep, 2010 16:37:05

Message: 7 of 19



I think t is time no negative but i don't sure

thank you for your reply

Subject: <<<<root>>>>help ME Plz

From: Walter Roberson

Date: 10 Sep, 2010 16:52:01

Message: 8 of 19

On 10/09/10 11:37 AM, tavirapak wrote:
>
>
> I think t is time no negative but i don't sure

There are 4 non-negative solutions to your equation. You can look for
them "near" 0, 1/2, 1, 3/2

Subject: <<<<root>>>>help ME Plz

From: tavirapak

Date: 10 Sep, 2010 17:14:05

Message: 9 of 19

So this equation can't solve.
Can you advice to me for solve that.

Subject: <<<<root>>>>help ME Plz

From: Matt Fig

Date: 10 Sep, 2010 17:17:23

Message: 10 of 19

Which can be seen by doing the following (there is a hint in there!).

f = @(t) 10*(exp(-t)).*sin(2*pi*t) - 2;
x = 0:.1:10;
plot(x,f(x),'b',[0 10],[0 0],'r')

Subject: <<<<root>>>>help ME Plz

From: tavirapak

Date: 10 Sep, 2010 17:42:18

Message: 11 of 19

"Matt Fig" <spamanon@yahoo.com> wrote in message <i6dp72$1c8$1@fred.mathworks.com>...
> Which can be seen by doing the following (there is a hint in there!).
>
> f = @(t) 10*(exp(-t)).*sin(2*pi*t) - 2;
> x = 0:.1:10;
> plot(x,f(x),'b',[0 10],[0 0],'r')

Oh, thank you very much

Subject: <<<<root>>>>help ME Plz

From: tavirapak

Date: 10 Sep, 2010 17:57:09

Message: 12 of 19

now, i can't show when t(axis x) at 2(axis y)

What can i do?

Subject: <<<<root>>>>help ME Plz

From: Matt Fig

Date: 10 Sep, 2010 18:13:10

Message: 13 of 19

"tavirapak " <belove_ng@hotmail.com> wrote in message <i6drhl$4fm$1@fred.mathworks.com>...
> now, i can't show when t(axis x) at 2(axis y)
>
> What can i do?

I have no idea what that means. You need to re-read all the suggestions which have been given to you in this thread. Notice I did not plot I(t) but f(t). How are I and f related????

Subject: <<<<root>>>>help ME Plz

From: tavirapak

Date: 10 Sep, 2010 18:21:23

Message: 14 of 19

i do this

t = 0:.1:10;
i = 10*(exp(-t)).*sin(2*pi*t) ;
plot(t,i,'g')

now i think answer is multi-root

Subject: <<<<root>>>>help ME Plz

From: Matt Fig

Date: 10 Sep, 2010 19:02:12

Message: 15 of 19

"tavirapak " <belove_ng@hotmail.com> wrote in message <i6drhl$4fm$1@fred.mathworks.com>...
> now, i can't show when t(axis x) at 2(axis y)
>
> What can i do?

I have no idea what that means. You need to re-read all the suggestions which have been given to you in this thread. Notice I did not plot I(t) but f(t). How are I and f related????

Subject: <<<<root>>>>help ME Plz

From: tavirapak

Date: 10 Sep, 2010 19:22:09

Message: 16 of 19

t = 0:.1:10;
i = 10*(exp(-t)).*sin(2*pi*t) ;
plot(t,i,'g')


for t = 0:.1:10
    i = 10*(exp(-t)).*sin(2*pi*t);
    if i~=2
       fprintf('t = %g\n',t)
    end
end

i want to show t at i=2 but it show all. Should i modify where

Subject: <<<<root>>>>help ME Plz

From: Walter Roberson

Date: 10 Sep, 2010 20:07:33

Message: 17 of 19

On 10-09-10 02:22 PM, tavirapak wrote:
> t = 0:.1:10;
> i = 10*(exp(-t)).*sin(2*pi*t) ;
> plot(t,i,'g')
>
>
> for t = 0:.1:10
> i = 10*(exp(-t)).*sin(2*pi*t);
> if i~=2
> fprintf('t = %g\n',t)
> end
> end
>
> i want to show t at i=2 but it show all. Should i modify where

You cannot do that. With the set of t values you are using, "i" will never
become 2 . There *will* be adjacent t values with one corresponding "i" value
less than 2 and the other corresponding "i" value greater than 2, but none of
those t values will ever have a corresponding "i" value of exactly 2.

The solutions for I=2 are fundamentally irrational numbers.

Subject: <<<<root>>>>help ME Plz

From: Walter Roberson

Date: 10 Sep, 2010 21:14:35

Message: 18 of 19

On 10-09-09 11:13 AM, tavirapak wrote:
> Sorry...I foget So...
>> > I = 10*(exp(-t))*sin(2*pi*t)
>> > > Where t is in seconds. Determine all values of t such that. I=2
>
> THANK YOU VERRY MUCH

I'll give you a hint:

If you have f(x) = c for some constant c, then by simple algebra you can
rewrite that as f(x) - c = 0 . Therefore any such problem can be easily
rewritten as trying to solve for the value that will make an expression
evaluate to 0. The process of finding the value that will make the expression
zero is known as "finding the zero" of the expression.

To determine which Matlab functions might be suitable for finding zeros, you
can go to the Matlab command prompt and give the command

lookfor zero

Now look at the summary description on each of the lines. If you see a
function that looks potentially useful, read the more detailed documentation
for that particular function.

Subject: <<<<root>>>>help ME Plz

From: Walter Roberson

Date: 13 Sep, 2010 20:19:41

Message: 19 of 19

On 10-09-10 03:07 PM, Walter Roberson wrote:
> On 10-09-10 02:22 PM, tavirapak wrote:
>> t = 0:.1:10;
>> i = 10*(exp(-t)).*sin(2*pi*t) ;
>> plot(t,i,'g')
>>
>>
>> for t = 0:.1:10
>> i = 10*(exp(-t)).*sin(2*pi*t);
>> if i~=2
>> fprintf('t = %g\n',t)
>> end
>> end
>>
>> i want to show t at i=2 but it show all. Should i modify where
>
> You cannot do that. With the set of t values you are using, "i" will
> never become 2 . There *will* be adjacent t values with one
> corresponding "i" value less than 2 and the other corresponding "i"
> value greater than 2, but none of those t values will ever have a
> corresponding "i" value of exactly 2.
>
> The solutions for I=2 are fundamentally irrational numbers.

Hint:

  F = @(t) 10*(exp(-t)).*sin(2*pi*t) - 2;
  fzero(F,3/2)

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