Invalid Initial Conditions Error

I keep getting an invalid initial condition error and I don't have enough experience using MATLAB to properly debug.

Running this code,

syms x(t) y(t) z(t) Dx Dy Dz Dx_i Dy_i Dz_i;
Dx = dsolve(diff(x, t) == x);
Dy = dsolve(diff(y, t) == y);
Dz = dsolve(diff(z, t) == z);
Dx_i = diff(x(0)) == 0;
Dy_i = diff(y(0)) == 0;
Dz_i = diff(z(0)) == 0;
cond = [x(0) == 1, y(0) == -1, z(0) == 1, Dx_i, Dy_i, Dz_i];
disp('Exercise 1c')
[x(t), y(t), z(t)] = dsolve([diff(x, t, 2) == 1*Dx - x - y, diff(y, t, 2) == -x - 1*Dy - y - 1*Dz, diff(z ,t ,2) == -9*z], cond)

As per as I am able to in regards to this questions,

I keep getting the error,

Error using mupadengine/feval_internal
Invalid initial conditions.

Error in dsolve>mupadDsolve (line 334)
T = feval_internal(symengine,'symobj::dsolve',sys,x,options);

Error in dsolve (line 203)
sol = mupadDsolve(args, options);

Error in Systems_of_Equations (line 20)
[x(t), y(t), z(t)] = dsolve([diff(x, t, 2) == 1*Dx - x - y, ...

Answer 1

[Suresh Maddina]

Hi, it is my understanding that you are trying to solve the system of differential equations in (c) (equation on the above image) using the initial conditions in (d)

The dsolve is incorrectly used with Dx, Dy, Dz terms. The initial conditions Dx, Dy, Dz are incorrectly defined. You may try the following

syms x(t) y(t) z(t) Dx_i Dy_i Dz_i;

Dx_i = diff(x, t);

Dy_i = diff(y, t);

Dz_i = diff(z, t);

cond = [x(0) == 1, y(0) == -1, z(0) == 1, Dx_i(0)==0, Dy_i(0)==0, Dz_i(0)==0];

[x(t), y(t), z(t)] = dsolve([diff(x, t, 2) == 1*diff(x,t) - x - y, diff(y, t, 2) == -x - 1*diff(y,t) - y - 1*diff(z,t), diff(z ,t ,2) == -9*z], cond)

Please refer to the dsolve examples here https://www.mathworks.com/help/symbolic/dsolve.html#bve5m8o-4