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Problem finding set of values for exponential function

Forum|Forum|12 years ago
November 28, 2013
5 replies
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Hello everyone.
I am having some trouble finding a set of values of y corresponding to x for the following equation.
y=exp(x+4y)

I would really appreciate any help i can get. Thanks.

1_mathcad1-xmcd.zip

Other

Best answer by AlanStevens

I agree with Werner. I find the easiest way to think about this is to take logs:

Alan

V

ValeryOchkov

24-Ruby IV

Sorry, may be I do not understand some one, but where is the problem:

W

Werner_E

25-Diamond I

Valery Ochkov wrote:
Sorry, may be I do not understand some one, but where is the problem:

Thats the problem:

28.11.png

Those two values are demanded to be the same.

W

Werner_E

25-Diamond I

You demand the impossible (assuming you expect real values).

You will only get solutions (two for each value of x) for x <= -2.383.

You may get them using a solve block.

W

Werner_E

25-Diamond I

Here is a sheet that should help and an animation demonstrating why values of x must be smaller than -2.383..

I still think that number should be -1-ln(4) and Mathcad allows for slightly bigger values only because of numerical inaccuracies in the solve block - anybody any comments on that?

A more detailled video (higher FRAME number) was uploaded here: http://communities.ptc.com/videos/4498

EDIT: Added 3D-plot

Answer

19-Tanzanite

I agree with Werner. I find the easiest way to think about this is to take logs:

Alan

W

Werner_E

25-Diamond I

Fine approach!

So -1-ln(4) definitely is the largest value allowed for x.

P

ptc-5403048

Author

1-Visitor

Thanks! Iogs seems to be doing the job.
However what i ultimately need is a set of values of y corresponding to x.

I tried your method and but what i managed was to get a set of values of x corresponding to y.
Is there any way i can extract this information from the graph? Thanks.

1_e-xmcd.zip

A

AlanStevens

19-Tanzanite

Do you mean something like the attached? Bear in mind that you won't get real values of y if your values of x are greater than -1-ln(4).

Alan

1_eb-xmcd.zip

W

Werner_E

25-Diamond I

Again the guess value y:=1 will give you the bigger of the two solutions while a smaller guess like 0.01 will yield the solution for y which is closer to zero.

P

ptc-5403048

Author

1-Visitor

I'm trying to apply this concept to another equation(same form as above). I don't know what i'm doing wrong but i'm not getting the range of values of y that i'm supposed to get.
I also plotted the equation using Mathlab just to verify. I'll be grateful if you could have a look at it. Thanks.