runge-Kutta problem command ode45

I try to solve an equation with the command ode45 b ut I have an error, this is my code:

tspan = [0 5];

y0 = 100;

Q = 30, b = 0.2;

[t,y,x] = ode45(@(tspan,y,x) Q-b*x*y, t, y0);

plot (t,y,x,'-o')

and the error:

Q =

30

Not enough input arguments.

Error in rungrkutta_y>@(tspan,y,x)Q-b*x*y (line 4)

[t,y,x] = ode45(@(tspan,y,x) Q-b*x*y, t, y0);

Error in odearguments (line 90)

f0 = feval(ode,t0,y0,args{:}); % ODE15I sets args{1} to yp0.

Error in ode45 (line 115)

odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);

Error in rungrkutta_y (line 4)

[t,y,x] = ode45(@(tspan,y,x) Q-b*x*y, t, y0);

how can I solve it?

Answer 1

[Star Strider]

I have no idea what you want to do, or what ‘x’ and ‘y’ are.

This will get you closer to an actual result:

tspan = [0 5];
y0 = [100; 0];
Q = 30; 
b = 0.2;

[t,y] = ode45(@(t,y) [y(1); Q-b*y(1)*y(2)], tspan, y0); 

figure
plot (t,y,'-o')

.

[Alvaro Mª Zumalacarregui Delgado]

I want to represent the solution of this system with the command ode45:

dx/dt = P-axy

dy/dt = Q-bxy

but I don´t know how to do it exactly

[Star Strider]

Try this:

tspan = [0 5];
y0 = [100; 0];                                                      % Supply The Correct Initial Value For ‘y’
Q = 30; 
b = 0.2;
a = 0.1;                                                            % Not Supplied
P = 10;                                                             % Not Supplied


[t,y] = ode45(@(t,y) [P-a*y(1)*y(2); Q-b*y(1)*y(2)], tspan, y0);    % x=y(1),  y=y(2)

figure
plot (t,y,'-o')
grid
legend('x(t)', 'y(t)')

I created values for the missing scalars to test the code. You will need to supply the correct values.

[Alvaro Mª Zumalacarregui Delgado]

I think that it will work, thank you!

[Star Strider]

As always, my pleasure!

Answer 2

[Stephan]

tspan = [0 5];
y0 = 100;
Q = 30;
b = 0.2;
[t,y] = ode45(@(t,y)Q-b*t*y, tspan, y0); 
plot (t,y,'-o')