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Math Help - 2nd Order Non-Homogeneous DE

  1. #1
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    2nd Order Non-Homogeneous DE

    Firstly Sorry for all the threads, but i have a test on DE's quite soon and i want to clarify as much as possible really without getting on my tutors bad books(i.e. annoying the hell out of her )

    I can solve this sort

    y'' + Ay' + B' = Ax^2 + Bx^2 + C

    However this question is like this

    y'' - y = 2cosx - 2sinx

    I haven't a clue how to solve the particular when it ends like that. With my first example you just say Ax^2 + Bx^2 + C and work out the constants, but with this i am unsure.

    I have worked out the y_{c}(x) part

    and get

    y_{c}(x) = c_{1}e^{-x}+c_{2}e^x

    But i haven't a clue where to start with y_{p}(x)

    Please help, thanks.
    Last edited by Rapid_W; June 4th 2009 at 03:17 PM.
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  2. #2
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    Quote Originally Posted by Rapid_W View Post
    Firstly Sorry for all the threads, but i have a test on DE's quite soon and i want to clarify as much as possible really without getting on my tutors bad books(i.e. annoying the hell out of her )

    I can solve this sort

    y'' + Ay' + B' = Ax^2 + Bx^2 + C

    However this question is like this

    y'' - y = 2cosx - 2sinx

    I haven't a clue how to solve the particular when it ends like that. With my first example you just say Ax^2 + Bx^2 + C and work out the constants, but with this i am unsure.

    I have worked out the y_{c}(x) part

    and get

    y_{c}(x) = c_{1}e^{-x}+c_{2}e^x

    But i haven't a clue where to start with y_{p}(x)

    Please help, thanks.
    Use y_p = A \cos (x) + B \sin (x).
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  3. #3
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    Thanks ever so much!

    If for example it was = to 10cosx, what would my plan of action be then?
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  4. #4
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    Quote Originally Posted by Rapid_W View Post
    Thanks ever so much!

    If for example it was = to 10cosx, what would my plan of action be then?
    Use y_p = A \cos (x) + B \sin (x).
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  5. #5
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    The easiest way of finding y_p when function on the right is a sum of trigonometric functions is to try

    y_p = A \sin x + B \cos x

    and equate coefficients.

    I.e.

    y_{p}' = A \cos x - B \sin x

    y_{p}'' = - (A \sin x + B \cos x) = - y_p

    so substituting for y_{p}'' in your equation gives

    y_{p}'' - y_{p} = -2 y_{p} = 2 \cos x - 2 \sin x

    \Leftrightarrow -A \sin x - B \cos x = \cos x - \sin x.

    I.e. A = 1 and B = -1 .

    So the final solution given by y = y_c + y_p is

     \boxed{ y= c_1 e^{-x} + c_2 e^{x} + \sin x - \cos x } .
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    Sorry, Mr. Fantastic's posts weren't there when I started posting.
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  7. #7
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    Quote Originally Posted by the_doc View Post
    The easiest way of finding y_p when function on the right is a sum of trigonometric functions is to try

    y_p = A \sin x + B \cos x

    and equate coefficients.

    I.e.

    y_{p}' = A \cos x - B \sin x

    y_{p}'' = - (A \sin x + B \cos x) = - y_p

    [snip]
    @OP: I will add that this type of particular solution will not work when the non-homogeneous bit of the DE contains \cos (\lambda x) or \sin (\lambda x) terms where \lambda is a solution to the auxillary equation.
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  8. #8
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    Thanks i know your answer is correct, but my arithmatic is wrong and i can't work out where

    y_{p}(x)=Asinx+Bcosx
    y'=Acosx-Bsinx
    y''=-Asinx - Bsinx
    so
    (-A-B)sinx+(A-B)cosx
    giving
    -A-B=-2
    A-B=2
    and i keep getting B=0 and A=-2.

    i'll run through what you have done again but i can't work out the jump where you have used the iff symbol
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  9. #9
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    Worked out where i've gone wrong. I used y', where i should have used y. I've been awake too long lol, thanks everyone for all your help, i think i've cracked this type of DE now!
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  10. #10
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    Hit another obstacle, sorry guys...

    y''-7y'+10y=9e^{2x}

    y_{c}(x) = c_{1}e^{2x}+c_{2}e^{5x}

    and i took what i though was a logical step for y_{p}(x)

    y_{p}(x)=Ae^{2x}

    however when i simplified all the substituting i ended up with 0Ae^{2x}
    giving 0A=9

    which is obviously no help because i highly doubt y_{p}(x) can equal \infty.

    Thanks.
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  11. #11
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    Try:

    y_p (x) = A \,x \,e^{2x}.

    When the non-homogeneous part is a multiple of a part of your complementary function (in this case e^{2x}) then you should try x times it.

    In general you can always use the method of 'Variation of parameters' but it can be alot of work!
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  12. #12
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    Thanks, i had just started to try that, there is a lot of work, but i get it down to this

    y_{p}=Axe^{2x}
    y_{p}'=2Axex^{2x} + Ae^{2x}
    y_{p}''=4Axe^{2x} + 4Ae^{2x}

    and plugged back in give

    -3Ae^{2x}

    So -3A = 9
    A=-3

    so y_{p}(x)=-3xe^{2x}

    Now, i'm not fully confident with this as Wolfram Alpha lists the particular interval as -e^{2x}(3x+1)

    y''-7y'+10y=9e^(2x) - Wolfram|Alpha

    I understand why the 3x is in the bracket, but where has the +1 come from?
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  13. #13
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    Quote Originally Posted by Rapid_W View Post
    Thanks, i had just started to try that, there is a lot of work, but i get it down to this

    y_{p}=Axe^{2x}
    y_{p}'=2Axex^{2x} + Ae^{2x}
    y_{p}''=4Axe^{2x} + 4Ae^{2x}

    and plugged back in give

    -3Ae^{2x}

    So -3A = 9
    A=-3

    so y_{p}(x)=-3xe^{2x}

    Now, i'm not fully confident with this as Wolfram Alpha lists the particular interval as -e^{2x}(3x+1)

    y''-7y'+10y=9e^(2x) - Wolfram|Alpha

    I understand why the 3x is in the bracket, but where has the +1 come from?
    y_p = -3x e^{2x} is the correct particular solution.

    If you add the Wolfram Alpha particular solution to y_c you will see why the +1 is irrelevant.
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  14. #14
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    Quote Originally Posted by Rapid_W View Post
    Firstly Sorry for all the threads, but i have a test on DE's quite soon and i want to clarify as much as possible really without getting on my tutors bad books(i.e. annoying the hell out of her )

    I can solve this sort

    y'' + Ay' + B' = Ax^2 + Bx^2 + C

    However this question is like this

    y'' - y = 2cosx - 2sinx

    I haven't a clue how to solve the particular when it ends like that. With my first example you just say Ax^2 + Bx^2 + C and work out the constants, but with this i am unsure.

    I have worked out the y_{c}(x) part

    and get

    y_{c}(x) = c_{1}e^{-x}+c_{2}e^x

    But i haven't a clue where to start with y_{p}(x)

    Please help, thanks.
    Alternatively, we could take the Laplace transform of both sides. Let  \mathcal{L} \{y(x) \} = Y(s) . We have y'' - y = 2 \cos x - 2 \sin x. Then  \mathcal{L} \{y'' - y \} = \mathcal{L} \{2 \cos x - 2 \sin x \} . Solve for  Y(s) . Then take inverse Laplace transforms to get  y(x) .

    We can use this method because this is a second order differential equation with constant coefficients. The point of taking Laplace transforms is to convert the differential equation into an "algebraic" one.
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