# Solution of Coupled Second Order ODEs with Boundary Conditions

   D[Tu1[x], {x, 2}] qu - Tu1[x] (fu0 + I w p) == Tou[x] f1

D[T1[x], {x, 2}] q - T1[x] (f0 - b g0 + I w p)==
To[x] (f1 - b g1) - g1


For T1 , Range[0, L/2]

For Tu1, Range[L/2, La/2]

Here, there are two differential equations with coupled boundary conditions;

T1'==0
Tu1[La/2] == 0
Tu1[L/2] == T1[L/2]
a4 Tu1'[L/2] == T1'[L/2]


And also,

Tou[x_] := E^(a3 x) c2 + E^(-a3 x) c3
To[x_] := 2 c1 Cosh[x a1] + a2


I know all constants in the equations.

I tried following code, but I coud not get answer, " error: not the same shape "

{T1, Tu1} = {T1, Tu1} /.
DSolve[{D[T1[x], {x, 2}] q - T1[x] (f0 - b g0 + I w p) -
To[x] (f1 - b g1) + g1 == 0,
D[Tu1[x], {x, 2}] qu - Tu1[x] (fu0 + I w p) - Tu[x] f1 == 0,
T1' == 0, Tu1[L/2] == T1[L/2], a4 Tu1'[L/2] == T1'[L/2],
Tu1[La/2] == 0}, {T1, Tu1}, x]


How can I find solution of differential equations?

• have you tried ?DSolve Apr 8 '14 at 8:00
• I have tried solving with Green's function technique, but failed Apr 8 '14 at 8:01
• what is Tou in the first equation? (which should have == instead of =) Apr 8 '14 at 8:07
• Sorry mate, I fixed now. Apr 8 '14 at 8:12
• no you haven't :-) Apr 8 '14 at 8:17

This is not an answer to your question but a copy and paste from the documentation which you might find inspiring:

Boundary Value Problems with Parameters

In many of the applications where boundary value problems arise, there may be undetermined parameters, such as eigenvalues, in the problem itself that may be a part of the desired solution. By introducing the parameters as dependent variables, the problem can often be written as a boundary value problem in standard form.

For example, the flow in a channel can be modeled by

$$f'''-R\,((f')^2-f f'')+R\, a==0$$

where $R$ (the Reynolds number) is given, but a is to be determined.

To find the solution $f$ and the value of a, just add the equation a'==0.

This solves the flow problem with R==1 for f and a, plots the solution f and returns the value of a.

 Block[{R = 1},
sol = NDSolve[{f'''[t] - R ((f'[t])^2 - f[t] f''[t]) + R a[t] == 0,
a'[t] == 0, f == f' == f' == 0, f == 1}, {f, a}, t];
Column[{Plot[f[t] /. First[sol], {t, 0, 1}],
a /. First[sol]}]] 