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November 18th, 2012, 04:06 PM   #1
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Finding the Particular Solution of diff eqn

dy/dx = 8x^3 -9x^2 + 4 and y =5, when x = 0


wanted to see this worked out in full

ty
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November 18th, 2012, 04:10 PM   #2
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Re: Finding the Particular Solution of diff eqn

Hi,

Do you know what is the antiderivative of , , ...?
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November 18th, 2012, 04:25 PM   #3
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Re: Finding the Particular Solution of diff eqn

yes but am new to these

x^3 = 3x^2 / 2
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November 18th, 2012, 04:52 PM   #4
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Re: Finding the Particular Solution of diff eqn

New to these? Who are you kidding? It is like months you are asking for problems of that sort, actually this one is simpler than many you posted and that mostly Mark has guided you to the solution. Are you just copying any question that you have in your book to solve or are you trying to solve them before and then post? Learn to sweat a bit on your paper sheet, MarkFL will not always be by your side! (perhaps can we adopt a MarkFL? ) I don't want you to take it bad, I am just giving a honest advise, which is to try solving problem by yourself first (and then post if you are REALLY stuck)...and this one I am sure you could have solved if you had really wanted? No?
For the answer you know the antiderivative of x^2 is x^3/3 (I am not using latex since you seem not to like it ) of x^3 is x^4/4...etc...(try to remember it for now... )
so y=(8/4)x^4-(9/3)x^3+(4/1)x+cte so y=2x^4-3x^3+4x+cte. Since you want y(0)=5 this means, cte=5.
cheers.

PS: To do latex just write exactly what you usually do but press latex on the little box above. What's complicate about it? What about to celebrate your 700 posts you start doing this?
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November 18th, 2012, 05:05 PM   #5
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Re: Finding the Particular Solution of diff eqn

Thx Dougy
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November 18th, 2012, 05:17 PM   #6
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Re: Finding the Particular Solution of diff eqn

Great! Sorry if I have been a bit harsh, I am glad you understood I am trying to go in a constructive way, not the opposite... it is very good that you are studying all these exercises at the end! Now think, could you solve another problem similar to this one? For instance could you do the equivalent problem
and we want y to be equal to 5 at x=1, meaning y(1)=5? If you can do this, this means that you can do any problem like this with powers of x and when you see this in your exam that will be a peace of cake!
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November 18th, 2012, 06:35 PM   #7
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Re: Finding the Particular Solution of diff eqn

I think this is wrong. Maybe u can point out my errors

6x^6/6 + x^5/5 -2x^3/3 + 4x^2/2 + Csub1


?
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November 18th, 2012, 07:02 PM   #8
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Re: Finding the Particular Solution of diff eqn

Almost good! (but the fact that you still don't want to use latex ) You did a mistake in the coefficient in , it should be Do you see it? All the rest looks good!
What about the value of the constant? y(1)=5, so what is Csub1? Plug x=1 and y(1)=5 and you will obtain an equation and you can solve for Csub1 and obtain its value.

For the Latex thing, so you write things like y=5x^5+3x^2 for instance, now you write the exact same thing but inside the latex...you press the little box "latex" (I guess you can find it ) and write inside, not that complicate right? For each equation you have to start a new latex box and in between you write as you do normally.
You want a sin, write \sin, a cos write \cos, an integral write \int, you want to write x/3 write \frac{x}{3} so like [ latex]\frac{x}{3}[/latex]= or [ latex]x^3[/latex]=

edit: Actually I did not pay enough attention, all coefficients are wrong! I don't have time now to explain but you still have not understood this...try to work more on it and find your mistakes. Even if you have the correct solution to the other problem you will do wrong at your quiz on such questions as long as you don't know how to do them YOURSELF. We can discuss this later, try to work on it..you are not that far to understand!! Courage!
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November 18th, 2012, 07:26 PM   #9
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Re: Finding the Particular Solution of diff eqn

ok I will review in my book at work the problem again from start to finish.

Also, I am going to post a new problem This one is an approximation problem. I will show as much work as I can


ps...I will post it with LATEX or at least try
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November 18th, 2012, 08:07 PM   #10
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Re: Finding the Particular Solution of diff eqn

Here we go ..from start to finish...

= find particular solution when y = 5, when x =0

dx

y =

y =

now when y = 5 and x = 0 C must equal 5

?
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