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Second order ODE question.

I don't know what's wrong with me. I'm in my second year of uni and I feel embarrassed to ask this question, because I SHOULD know it.

i have to solve this second order ODE:

d2ydx2+dydc2y=2 \frac{d^2y}{dx^2} +\frac{dy}{dc} -2y = -2

y(0)=1 y(0) = 1 y(1)=2y(1)=2 find y(0.2) y(0.2)


the thing that's annoying me is that 2.
What do we do with it?
Here's what I've done so far:

associated homogeneous equation: y+y2y=0 y'' + y'-2y = 0
so m=2,1 m=-2, 1

complementary solution: Yc=C1ex+C2e2xY_c = C_1e^x + C_2e^{-2x}

now for the particular solution: Yp=AY_p=A so Yp=yp=0Y_p' = y_p''=0
and substituting that in i get A =1 so Yp=1 Y_p =1

to satisfy the initial conditions we want,
C1+C2=1 C_1 + C_2 = 1
C1e+C2e2=2 C_1e + C_2e^{-2} = 2

so C2=e21e3 C_2 = \frac{e^2}{1-e^3} and 1e21e31-\frac{e^2}{1-e^3}

and now y=C1ex+C2e2x+1 y = C_1e^x + C_2e^{-2x} + 1

y(0.2)=3.1446 y(0.2) = 3.1446 but the answer is 1.49588


Any help will be greatly appreciated!
(edited 12 years ago)
Reply 1
Original post by FakeName12345
I don't know what's wrong with me. I'm in my second year of uni and I feel embarrassed to ask this question, because I SHOULD know it.

i have to solve this second order ODE:

d2ydx2+dydc2y=2 \frac{d^2y}{dx^2} +\frac{dy}{dc} -2y = -2

y(0)=1 y(0) = 1 y(1)=2y(1)=2 find y(0.2) y(0.2)


the thing that's annoying me is that 2.
What do we do with it?
Here's what I've done so far:

associated homogeneous equation: y+y2y=0 y'' + y'-2y = 0
so m=2,1 m=-2, 1

complementary solution: Yc=C1ex+C2e2xY_c = C_1e^x + C_2e^{-2x}

now for the particular solution: Yp=AY_p=A so Yp=yp=0Y_p' = y_p''=0
and substituting that in i get A =1 so Yp=1 Y_p =1

to satisfy the initial conditions we want,
C1+C2=1 C_1 + C_2 = 1
C1e+C2e2x=2 C_1e + C_2e^{-2x} = 2

so C2=e21e3 C_2 = \frac{e^2}{1-e^3} and 1e21e31-\frac{e^2}{1-e^3}

and now y=C1ex+C2e2x+1 y = C_1e^x + C_2e^{-2x} + 1

y(0.2)=3.1446 y(0.2) = 3.1446 but the answer is 1.49588


Any help will be greatly appreciated!

before working out the simultaneuous equation, you need to say general=CF +PI and THEN work out the value of the constants - you have forgotten the PI which you worked out earlier.
Reply 2
Original post by anshul95
before working out the simultaneuous equation, you need to say general=CF +PI and THEN work out the value of the constants - you have forgotten the PI which you worked out earlier.


Thanks for replying.
ok I did that, but i get 0.4789 which is still wrong.
(edited 12 years ago)
Original post by FakeName12345

C1+C2=1 C_1 + C_2 = 1
C1e+C2e2=2 C_1e + C_2e^{-2} = 2



surely C1+C2=0C_{1}+C_{2}=0 because after you add the CF and PI you have y=C1ex+C2e2x+1y=C_{1}e^{x}+C_{2}e^{-2x}+1 and after you substitute y(0)=1y(0)=1 the one's cancel.
Reply 4
Original post by FakeName12345
Thanks for replying.
ok I did that, but i get 0.4789 which is still wrong.

you have misunderstood me- because you didn't take into account the extra 1
y=Ae^x+Be^2x+1

NOW use the intial conditions. You should find that you get:
A+B=0 and Ae^3+B=e^2

Edit: by the way I subbed y(0.2) and I got 1.21335...... I checked it on wolframalpha
(edited 12 years ago)
Reply 5
Original post by anshul95
you have misunderstood me- because you didn't take into account the extra 1
y=Ae^x+Be^2x+1

NOW use the intial conditions. You should find that you get:
A+B=0 and Ae^3+B=e^2

Edit: by the way I subbed y(0.2) and I got 1.21335...... I checked it on wolframalpha


ahh wicked!! You're amazing! Thank you so much!

oh damn btw, i meant y(0.5)

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