MATLAB: Help with the code

dsolvedsolve plus ode45MATLAB

Help, I have this code, where I wanted to use 'dsolve to get the answer, but it shows me an 'empty sym' like answer:
clear all
clc
disp('Se resolverá una ecuación integro - diferencial')
syms vel(t) t desp(t)
V = input('Ingrese el voltaje: ' );
l = input('Ingrese la longitud: ');
M = input('Ingrese la masa: ');
B = input('Ingrese la inducción magnética: ');
froz = input('Ingrese la fuerza de rozamiento: ');
eqn = diff(vel,t) == (1/M)*(((V*l*B)-(vel(t)*(l^2)*(B^2)))/(1+(2*int(vel, [0 t])))-froz);
%eqn = diff(vel,t) == 2*int(vel, [0 t])
vel(t) = dsolve(eqn, 'vel(0) = 0')
pretty(vel(t))
What can I do?

Best Answer

Matlab is having trouble to solve the equation like sir Walter suggests so i converted it into numerical solution: 
%SYMBOLIC TO NUMERICAL METHOD 
clear all
clc
disp('Se resolverá una ecuación integro - diferencial')
syms vel(t) 
V = input('Ingrese el voltaje: ' );
l = input('Ingrese la longitud: ');
M = input('Ingrese la masa: ');
B = input('Ingrese la inducción magnética: ');
froz = input('Ingrese la fuerza de rozamiento: ');
vars=vel(t)
eqn = diff(vel,t) == (1/M)*(((V*l*B)-(vel(t)*(l^2)*(B^2)))/(1+(2*int(vel, [0 t])))-froz);
%eqn = diff(vel,t) == 2*int(vel, [0 t])
% vel(t) = dsolve(eqn, 'vel(0) = 0')
% pretty(vel(t))
V = odeToVectorField(eqn)
M = matlabFunction(V,'vars', {'t','Y'})
interval = [0 10];  %time interval
y0 = 0;      %initial conditions
ySol = ode45(M,interval,y0);
tValues = linspace(interval(1),interval(2),1000);
yValues = deval(ySol,tValues,1); %number 1 denotes first solution likewise you can mention 2 & 3 for the next two solutions
plot(tValues,yValues)
%DIRECT NUMERICAL METHOD
V = input('Ingrese el voltaje: ' );
l = input('Ingrese la longitud: ');
M = input('Ingrese la masa: ');
B = input('Ingrese la inducción magnética: ');
froz = input('Ingrese la fuerza de rozamiento: ');
[t,x] = ode45(@(t,x)myod(t,x,V,l,M,B,froz),[0 10],0);
plot(t,x,'-om')
function dxdt = myod(t,x,V,l,M,B,froz)
dxdt =(1/M)*(((V*l*B)-(x(1)*(l^2)*(B^2)))/(1+(2*cumtrapz(x(1))))-froz);
end
An example of the solution graph produced:
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