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-   -   Separable equation constant generality. (http://www.physicsforums.com/showthread.php?t=524345)

shayaan_musta Aug25-11 02:27 PM

Separable equation constant generality.
 
Hello experts!

I need your point of view about the following,

Do you think is there any loss of generality if the arbitrary constant added when a separable equation is integrated is written in the form lnC rather than just C?
Do you think this would ever be convenient thing to do?
Is there any loss of generality if the integration constant is written in the form of C2? tanC? sinC? ex? sinhX? coshX?

Kindly clarify my concept in easiest way you can do. Use simple English, because my English is some weak.

Thanks experts.

HallsofIvy Aug25-11 03:57 PM

Re: Separable equation constant generality.
 
If C is an arbitrary (but positive) constant, then ln(C) is also an arbitrary constant. As long as you careful about the signs, there will be no problem.

Perhaps you are thinking about the situation where you integrate to get ln|y|= f(x)+ C. Yes, you could replace C with ln C' (or just use C again if there is no danger of confusion) to say that [itex]|y|= C'e^{f(x)}[/itex]. Equivalently, you could just say that taking exponentials of both sides of ln|y|= f(x)+ C to get [itex]|y|= e^{f(x)+ C}= e^Ce^{f(x)}= C'e^{f(x)}[/itex] where [itex]C'= e^C[/itex]. Of course, for C any real number, [itex]e^C> 0[/itex]. If we allow C' to be, instead, any real number, we can remove the absolute value sign.

shayaan_musta Aug26-11 03:43 AM

Re: Separable equation constant generality.
 
Quote:

Quote by HallsofIvy (Post 3469370)
If C is an arbitrary (but positive) constant, then ln(C) is also an arbitrary constant. As long as you careful about the signs, there will be no problem.

Perhaps you are thinking about the situation where you integrate to get ln|y|= f(x)+ C. Yes, you could replace C with ln C' (or just use C again if there is no danger of confusion) to say that [itex]|y|= C'e^{f(x)}[/itex]. Equivalently, you could just say that taking exponentials of both sides of ln|y|= f(x)+ C to get [itex]|y|= e^{f(x)+ C}= e^Ce^{f(x)}= C'e^{f(x)}[/itex] where [itex]C'= e^C[/itex]. Of course, for C any real number, [itex]e^C> 0[/itex]. If we allow C' to be, instead, any real number, we can remove the absolute value sign.

I have better understood your thinking.

So the conclusion of discussion can be that any integration constant we use in place of C, either it is lnC, C2, tanC, sinC, ex, sinhX, coshX. Answer of the separable equation will be same as it is on just simple C?

Am I right??

HallsofIvy Aug26-11 06:24 AM

Re: Separable equation constant generality.
 
A constant is a constant is a constant! It doesn't matter how you write it as long as it reduces to a number.

shayaan_musta Aug26-11 07:55 AM

Re: Separable equation constant generality.
 
Ok.
I have fully understood that A constant is always a constant. It doesn't matter how you write it

But as you said
Quote:

as long as it reduces to a number.
What does mean by above statement?

But in the constant list I have also shown that ex. What will you comment about ex? Is this can be written in the place of C or lnC, whatever I've listed?
I am asking this because I don't think so that ex is not a constant. Because it has a variable x as a superscript, contained in the separable equation.
Am I right? Can it be written as a constant too?


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