# Differential Equation options question?

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• Mar 28th 2014, 12:09 PM
canyouhelp
Differential Equation options question?
I tried my best on this question: Attachment 30551 Is my answer right? I'm just not sure because I keep getting formulas confused :/
• Mar 29th 2014, 06:57 PM
HallsofIvy
Re: Differential Equation options question?
No, your answer is not correct. If you want someone to say what you did wrong, then you will have to tell us what you did!
• Mar 30th 2014, 06:17 AM
canyouhelp
Re: Differential Equation options question?
Quote:

Originally Posted by HallsofIvy
No, your answer is not correct. If you want someone to say what you did wrong, then you will have to tell us what you did!

With this one I struggled A LOT..because there were so many variables and I kept getting them confused :/ I tested each equation to see if it fit the question, but it took me a while and I wasn't sure how to test them. Can you help me with this?
• Mar 31st 2014, 05:28 AM
hollywood
Re: Differential Equation options question?
Were you able to eliminate equation II? Why doesn't it work?

- Hollywood
• Mar 31st 2014, 06:00 AM
canyouhelp
Re: Differential Equation options question?
Quote:

Originally Posted by hollywood
Were you able to eliminate equation II? Why doesn't it work?

- Hollywood

Well, I think it's a red flag that it doesn't even have M, maximum performance, in it. Maybe that isn't a reason to eliminate it though?
• Mar 31st 2014, 06:26 AM
romsek
Re: Differential Equation options question?
Quote:

Originally Posted by canyouhelp
Well, I think it's a red flag that it doesn't even have M, maximum performance, in it. Maybe that isn't a reason to eliminate it though?

You're guessing.

What do you expect the value of \$P_L\$ to be as time goes to infinity?

What do you expect the derivative to approach as \$P_L\$ approaches this value?
• Mar 31st 2014, 07:39 AM
canyouhelp
Re: Differential Equation options question?
Quote:

Originally Posted by romsek
You're guessing.

What do you expect the value of \$P_L\$ to be as time goes to infinity?

What do you expect the derivative to approach as \$P_L\$ approaches this value?

I think as the value of PL will increase when time approaches infinity. Right? Actually wouldn't it increase and then decrease? and the derivative of PL as PL curves would be a constant decreasing equation?
• Mar 31st 2014, 11:42 AM
romsek
Re: Differential Equation options question?
Quote:

Originally Posted by canyouhelp
I think as the value of PL will increase when time approaches infinity. Right? Actually wouldn't it increase and then decrease? and the derivative of PL as PL curves would be a constant decreasing equation?

you are told \$M\$ is the maximum value \$P_L\$ will attain....
• Mar 31st 2014, 11:56 AM
canyouhelp
Re: Differential Equation options question?
Quote:

Originally Posted by romsek
you are told \$M\$ is the maximum value \$P_L\$ will attain....

Oh god, I'm an idiot. Okay so the value of PL will approach M as time approaches infinity. And I'm not sure what will happen with the derivative then. :/
• Mar 31st 2014, 12:22 PM
romsek
Re: Differential Equation options question?
Quote:

Originally Posted by canyouhelp
Oh god, I'm an idiot. Okay so the value of PL will approach M as time approaches infinity. And I'm not sure what will happen with the derivative then. :/

If a function becomes constant, say at \$P_L=M\$, then what value must the derivative go to?
• Mar 31st 2014, 01:57 PM
canyouhelp
Re: Differential Equation options question?
Quote:

Originally Posted by romsek
If a function becomes constant, say at \$P_L=M\$, then what value must the derivative go to?

Is P'(L)= 0? I think.. :/
• Mar 31st 2014, 05:16 PM
romsek
Re: Differential Equation options question?
Quote:

Originally Posted by canyouhelp
Is P'(L)= 0? I think.. :/

yes. Now there are two choices that satisfy these 2 conditions. You'll need to do a bit of work to figure out why one isn't correct.
• Mar 31st 2014, 05:29 PM
canyouhelp
Re: Differential Equation options question?
Quote:

Originally Posted by romsek
yes. Now there are two choices that satisfy these 2 conditions. You'll need to do a bit of work to figure out why one isn't correct.

Thank you so much! It took me forever but I separated all of them and eliminated the last one. I had to do the third one twice but it's derivative after separating doesn't equal 0. So it'd have to be option 2. I and II, right?
• Mar 31st 2014, 05:34 PM
romsek
Re: Differential Equation options question?
Quote:

Originally Posted by canyouhelp
Thank you so much! It took me forever but I separated all of them and eliminated the last one. I had to do the third one twice but it's derivative after separating doesn't equal 0. So it'd have to be option 2. I and II, right?

Our condition is that as \$P_L\$ approaches \$M\$, \$\dfrac{dP_L}{dt}\$ approaches 0.

Does II satisfy that?

Does it look like III satisfies that?

You don't have to do the separation to immediately rule out II and IV.
• Mar 31st 2014, 05:41 PM
canyouhelp
Re: Differential Equation options question?
Quote:

Originally Posted by romsek
Our condition is that as \$P_L\$ approaches \$M\$, \$\dfrac{dP_L}{dt}\$ approaches 0.

Does II satisfy that?

Does it look like III satisfies that?

You don't have to do the separation to immediately rule out II and IV.

Um. I had I and III to begin with and I was told that was wrong...
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