Trouble using dsolve function

34 次查看（过去 30 天）
Nitin
Nitin 2012-1-20
Hi,
I need help in solving a 2nd order diff equations. Since my equations are very long (almost 30 lines), I have written a similar code below to explain my question.
z= a*x+b*x^2+c+........; S =dsolve('D2y +5*z+ 2*x + 10*z^2','x')
Now since my z equation is very long I need to substitute z in the dsolve function. I tried using inline function by doing this
z = inline ('a*x+b*x^2+c+........')
but I am not sure if the dsolve program is taking the value of z. Its running though, and it says no explicit solution found and returns me an empty symbol. Am I doing the right thing or is there any other way to do it.

请先登录，再回答此问题。

采纳的回答

Walter Roberson
Walter Roberson 2012-1-20
z = a*x+b*x^2+c+........;
eqn = subs(sym('D2y +5*z+ 2*x + 10*z^2'), z, z);
S = dsolve(eqn, x);
Myself, I would probably toss in a simplify() around the subs() .
  2 个评论
Nitin
Nitin 2012-1-20
Hey Walter,
Thank you for your reply. I tried you subs and simplify function like this for my equation
eqn1 = simplify(subs(sym('(D2p1/((8.314)*(573.15)))+(((((p1+p2+p3)*Dp2-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*N1+(p2/(p1+p2+p3))*(-N1/r)-N2*(((p1+p2+p3)*Dp1-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)-(p1/(p1+p2+p3))*(-N2/r))/(1.7241e-05)+((((p1+p2+p3)*Dp3-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*N1+(-N1/r)*(p3/(p1+p2+p3))-(p1/(p1+p2+p3))*(-k*p1/(R*T)-N3/r)-N3*(((p1+p2+p3)*Dp1-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2))/(1.0642e-05))+(-N1/r)/(5.6164e-08)+((9.5703e-18)/((8.314)*(573.15)*(0.00001027)*(5.6164e-08))*((((p1+p2+p3)*Dp1-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*(p1+p2+p3)*(Dp1+Dp2+Dp3)+(p1/(p1+p2+p3))*(Dp1+Dp2+Dp3)*(p1+p2+p3)+(p1/(p1+p2+p3))*((Dp1+Dp2+Dp3))^2))'), N1, N1, N2, N2, N1, N1, N3, N3, N1));
But I get an error
??? Error using ==> sym.subs
Too many input arguments.
Can you show me the right syntax for this.
Thanks once again.
Walter Roberson
Walter Roberson 2012-1-20
The syntax I used is valid for the case of exactly one thing to substitute in. When you have multiple things to substitute in, you must form lists.
eq1 = simplify(subs(...., {'N1', 'N2', 'N3'}, {N1, N2, N3}))

请先登录，再进行评论。

更多回答（2 个）

Nitin
Nitin 2012-1-20
Hey Walter,
I fixed the problem. Many thanks for your help. Can you tell me one more thing. After I run my program, it gives me this message Warning: Empty equation In dsolve at 91 In secondorder at 92
can you tell me what this means. I searched on the matlab website and there is no such warnings listed.
thanks, Nitin
Nitin
Nitin 2012-1-20
Hey Walter,
This is my code
eqn1 = simplify(subs(sym('(D2p1/((8.314)*(573.15)))+(((((p1+p2+p3)*Dp2-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*N1+(p2/(p1+p2+p3))*(-N1/r)-N2*(((p1+p2+p3)*Dp1-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)-(p1/(p1+p2+p3))*(-N2/r))/(1.7241e-05)+((((p1+p2+p3)*Dp3-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*N1+(-N1/r)*(p3/(p1+p2+p3))-(p1/(p1+p2+p3))*(-k*p1/(R*T)-N3/r)-N3*(((p1+p2+p3)*Dp1-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2))/(1.0642e-05))+(-N1/r)/(5.6164e-08)+((9.5703e-18)/((8.314)*(573.15)*(0.00001027)*(5.6164e-08))*((((p1+p2+p3)*Dp1-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*(p1+p2+p3)*(Dp1+Dp2+Dp3)+(p1/(p1+p2+p3))*(Dp1+Dp2+Dp3)*(p1+p2+p3)+(p1/(p1+p2+p3))*((Dp1+Dp2+Dp3))^2))'), {N1 N1 N2 N2 N1 N1 N3 N3 N1}, {N1 N1 N2 N2 N1 N1 N3 N3 N1}));
eqn2 = simplify(subs(sym('(D2p2/((8.314)*(573.15)))+(((((p1+p2+p3)*Dp1-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*N2+(p1/(p1+p2+p3))*(-N2/r)-N1*(((p1+p2+p3)*Dp2-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)-(p2/(p1+p2+p3))*(-N1/r))/(1.7241e-05)+((p3/(p1+p2+p3))*(-N2/r)+N2*(((p1+p2+p3)*Dp3-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)-(p2/(p1+p2+p3))*(-k*p1/(R*T)-N3/r)-N3*(((p1+p2+p3)*Dp2-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2))/(6.5157e-06))+(-N2/r)/(3.3910e-08)+((9.5703e-18)/((8.314)*(573.15)*(0.00001027)*(3.3910e-08))*((((p1+p2+p3)*Dp2-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*(p1+p2+p3)*(Dp1+Dp2+Dp3)+(p2/(p1+p2+p3))*(Dp1+Dp2+Dp3)*(p1+p2+p3)+(p2/(p1+p2+p3))*((Dp1+Dp2+Dp3))^2))'), {N2 N2 N1 N1 N2 N2 N3 N3 N2},{N2 N2 N1 N1 N2 N2 N3 N3 N2}));
eqn3 = simplify(subs(sym('(D2p3/((8.314)*(573.15)))+(((((p1+p2+p3)*Dp1-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*N3+(-k*p1/(R*T)-N3/r)*(p1/(p1+p2+p3))-(p3/(p1+p2+p3))*(-N1/r)-N1*(((p1+p2+p3)*Dp3-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2))/(1.0642e-05)+((p2/(p1+p2+p3))*(-k*p1/(R*T)-N3/r)+N3*(((p1+p2+p3)*Dp2-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)-(p3/(p1+p2+p3))*(-N2/r)-N2*(((p1+p2+p3)*Dp3-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2))/(6.5157e-06))+(-k*p1/(R*T)-N3/r)/(1.9194e-08)+((9.5703e-18)/((8.314)*(573.15)*(0.00001027)*(1.9194e-08))*((((p1+p2+p3)*Dp3-(Dp1+Dp2+Dp3))/(p1+p2+p3)^2)*(p1+p2+p3)*(Dp1+Dp2+Dp3)+(p3/(p1+p2+p3))*(Dp1+Dp2+Dp3)*(p1+p2+p3)+(p3/(p1+p2+p3))*((Dp1+Dp2+Dp3))^2))'), {N3 N3 N1 N1 N3 N3 N2 N2 N3}, {N3 N3 N1 N1 N3 N3 N2 N2 N3}));
S = dsolve(eqn1, eqn2, eqn3, r)
This on running gives me empty equation warning. I used the subs above like you said to substitute for the values of N1 , N2 and N3. Can you tell me where I am wrong. Does N1, N2 and N3 not substitute in these equations ?
Thanks, Nitin

类别

在 Help Center 和 File Exchange 中查找有关 Function Creation 的更多信息

标签

产品

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by