The differential equation problem

i have a question but i dont solve it. i am beginner for matlab. i think that doesnt exist. do u have an idea?

[John D'Errico]

What doesn't exist?

Why not make an effort? Begin to learn, as that is the only way you will learn.

I'll give you a hint. There are many tools like ODE45 to solve simple ODEs of this form. But it appears here that you are expected to use expm. So try it.

[Bugra]

i try to solve for 2 days but i dont need use ode45 or ode23.i try this but graph is blank

A=[0 6 -1;6 2 -16;-5 20 -10];

x0=[1;1;1]

for t=0:5

    x=expm(A*t)*x0

   plot(t,x)

    hold on
end

Answer 1

[Star Strider]

You’re almost there, so I’ll provide the corrected loop. You have to index the output of a loop or you only get the last value. In this instance, it’s a bit more challenging, because the output is a column vector, so you have to provide for that, and index the output by rows.

Consider:

A=[0 6 -1;6 2 -16;-5 20 -10];
x0=[1;1;1];
t=linspace(0,5);
for k1 = 1:length(t)
    x(:,k1)=expm(A*t(k1))*x0;
end

figure(1)
plot(t,x)
grid

[Bugra]

Give the error. I dont understand length(t) is 1.

??? Subscripted assignment dimension mismatch.

Error in ==> task93 at 5 x(:,k1)=expm(A*t(k1))*x0;

[Star Strider]

I have no idea what could be wrong if you copied and pasted my code exactly as I posted it here. I tested it to be sure it worked before I posted it.

The ‘t’ vector I used here is a (1x100) row vector. Its length should be 100 and not be 1. Post all your code so I can see if I can find out what the problem is with it.

[Bugra]

Thank you. I use clc and clear all work your code.

[Star Strider]

My pleasure!

The .m-file I use to test my code for Answers clears my workspace automatically, so I sometimes forget to suggest it.