First, no need to use global variables. Using them is not a recommended practice. Second, just write your ODE inside the main1.
Write this in a script file
tspan= 0:0.1:50 ;
X0 = [5, -3] ;
[t, X] = ode45(@main1, tspan, X0);
plot(t,X)
And save the following function in the file "main1.m"
function homework1 = main1(t,X)
m = 1;
c = 0.02;
k1 = 1.25;
k3=0.05;
if t < 10
F = 0;
elseif t < 30
F = 3*cos(t.^2);
else
F = -10;
end
homework1= [X(2); -c/m*X(2)-k1/m*X(1)-k3/m*X(1).^3+F/m] ;
end
