WTF is the Solution to a derivative?! (1 Viewer)

[ensignkim113]

hey people,

I just did a 3 unit maths assessment. It gave you this

dM/Dt = -0.02M

They said to write the solution to that. I mean WTF is that supposed to mean!!??

it didn't mean to get M in terms of t because that was part b of the question.

 

[McLake]

If it's not M in terms of t or t in terms of M then I have NO idea ...

 

[Harimau]

Originally posted by ensignkim113
hey people,

I just did a 3 unit maths assessment. It gave you this

dM/Dt = -0.02M

They said to write the solution to that. I mean WTF is that supposed to mean!!??

it didn't mean to get M in terms of t because that was part b of the question.

It might be for an exponential growth and decay question. Is it?

 

[underthesun]

dM/dt = -0.02M

dt/dM = -50/M

t = -50Loge(M) + C

-t/50 + C = ln(M)
e^(-t/50+C) = M
M = e^(-0.02t) * e^(C)

since e^C is a constant, we let A = e^C
M = Ae^(-0.02t)

hence the expo.growth equation. My teacher made everyone in the class remember how to do this

 

[Lazarus]

Originally posted by underthesun
M = Ae^(-0.02t)

That looks like M in terms of t to me.

 

[ensignkim113]

Originally posted by underthesun
dM/dt = -0.02M

dt/dM = -50/M

t = -50Loge(M) + C

-t/50 + C = ln(M)
e^(-t/50+C) = M
M = e^(-0.02t) * e^(C)

since e^C is a constant, we let A = e^C
M = Ae^(-0.02t)

hence the expo.growth equation. My teacher made everyone in the class remember how to do this

nah, this isn't it, I know how to do that, it was in part b for like 3 marks. part a was 1 mark and it was simply write the solution to this equation.

 

[Harimau]

Originally posted by ensignkim113


nah, this isn't it, I know how to do that, it was in part b for like 3 marks. part a was 1 mark and it was simply write the solution to this equation.

Just wait for the solutions then.

 

[Rahul]

say if u differentiate f(x), u get f'(x). solutoins to f'(x) would be the x-intercepts on a graph of f'(x){when f'(x) = 0}, or when there r staionary points on the f(x) graph.

would that be it?

 

[Lazarus]

Probably.

But that would be solving dM/dt = 0 (i.e. f'(x) = 0)... instead of solving dM/dt = -0.02M. Hmm it could still be what they meant though; if so, the question should have been more explicit.

 

[wogboy]

Are you sure you weren't given any other information about M & t, or given particular value of M or t, to find dM/dt? If not, then that question is meaningless and should be taken out of the test. I'd certainly be pissed off if I got something like that in an exam. :mad1:

 

[underthesun]

wHoops, didnt read your question properly. Was stayhing up a bit latew..

anyways 1 mark question? seems like a one-liner answer question, so it looks like something out of the syllabus..

 

[ensignkim113]

hey you guys were right , the solution was A=Pe^kt but the best thing is i had that in part 2 so they gave me the mark for it anyway.