By using this site, you agree to our Privacy Policy and our Terms of Use. Close

Forums - General - Little Help Somebody?

Rath on 15 June 2008
Alright, I'm studying for a maths exam on monday and this question has me completely stumped.

Essentially its a seperation of variables question and it involves the equation

ds/dt=k(a-s)

Where 'a' is the initial value and the initial value is 50.

From what I have worked out this is

1/(50-s)ds = kdt

-ln(50-s) = kt
ln(50-s) = -kt
50-s = e^-kt
s = 50 - e^kt

However the correct answer is apparently s = 50(1 - e^kt) and I can't figure out how on earth they got that. Be very useful if somebody can help me out here.

Around the Network
WhiteDevil on 15 June 2008

Holy Hell *runs outta the thread*



Rath on 15 June 2008

It's only maths, its not actually evil or anything.



Menago KF on 15 June 2008

Ouch. Tough dudu my friend. I may be Polish and have tougher math at high school, but we operate more on numbers than equations, at least on the geography institute.



Huh. Who would've thought that beggining anew in my real life would coincide with starting anew on vgchartz?

Any day now, the dollar will be worth less than 2 zloty......any day now.....and my life savings will be in total jepordy ;(.

Menago KF on 15 June 2008

Although.....could you pm me about this? It doesn't seem bad to tell the truth but i need more info on what the hell dt and ds means in this equation.

Edit: Nevermind, only now i have realized that this is from the material i have worked on 3 years ago, totally outside my comprehension right now. Sorry.



Huh. Who would've thought that beggining anew in my real life would coincide with starting anew on vgchartz?

Any day now, the dollar will be worth less than 2 zloty......any day now.....and my life savings will be in total jepordy ;(.

Around the Network
Rath on 15 June 2008

ds/dt, its the equivalent notation of f'(t) basically. The derivative of the original equation (which I'm trying to find).



bobobologna on 15 June 2008

I think the problem is that the integral of 1/(50-s) isn't -ln(50-s). At least, I don't think it is. It's been a couple years since I've done this sort of math.



bobobologna on 15 June 2008

Oh, you know what, you are correct. But when you take the integral shouldn't a constant C be introduced? Maybe that's the problem? This problem is really bothering me now.