Hi i have made a separable differential equation now i want to put values of t at instant, equations are made but i down know how to put value

syms A(t) C(t) k

k=4;

ode1=diff(A,t)==-k*A

ode2=diff(C,t)==k*A

odes=[ode1;ode2]

S=dsolve(odes)

Asol(t)=S.A

Csol(t)=S.C

[Asol(t), Csol(t)]=dsolve(odes)

cond1=A(0)==5

cond2=C(0)==5

conds=[cond1;cond2]

[Asol(t),Csol(t)]=dsolve(odes,conds)

Answer 1

[Star Strider]

When you run your code, you create ‘Asol’ and ‘Csol’ as functions. So you only need to give them numeric arguments (here 3 arbitrarily) to get the result:

Result_1 = vpa(Asol(3))
Result_2 = vpa(Csol(3))

Result_1 =
0.000030721061766641048793411540894028

Result_2 =
9.9999692789382333589512065884591

[Arslan Ahmad]

thanks for your reply i have used function now and its still not giving any numerical values but values in exponential form

function [A,C]=absent(A0,C0,k,t)

syms A1(t) C1(t) k

ode1=diff(A1,t)==-k*A1;

ode2=diff(C1,t)==k*A1;

odes=[ode1;ode2];

S=dsolve(odes);

Asol(t)=S.A1;

Csol(t)=S.C1;

[Asol(t), Csol(t)]=dsolve(odes);

cond1=A1(0)==A0;

cond2=C1(0)==C0;

conds=[cond1;cond2];

[Asol(t),Csol(t)]=dsolve(odes,conds);

A = vpa(Asol(t))

C = vpa(Csol(t))

end

[Arslan Ahmad]

the output is

A =

5.0*exp(-1.0*k*t)

C =

10.0 - 5.0*exp(-1.0*k*t)

[Birdman]

Run my code and see the result.

[Star Strider]

If you want a numeric result, you need to use a numeric argument to the functions.

See my Answer, and Walter Roberson’s extension of it.

[Birdman]

Mine is also numeric substitution(t is substituted). After it, ASol and CSol become numeric vectors.

[Star Strider]

My code takes advantage of the original code creating functions, so you only need to deal with them as straightforward functions. It is much simpler and more efficient.

[Walter Roberson]

vpa() and subs() and calling the symbolic function with numeric values will all produce symbolic results. Those symbolic results can increasingly be used in place of numeric results, but not in all contexts.

To get a numeric result, apply double() to the symbolic result. Like double(Asol(1.234))

Answer 2

[Walter Roberson]

For example:

t_values = linspace(0, 10, 100);
plot(t_values, Asol(t_values))