MATLAB: Can you help me please !

solving a system of second order differential equations

I run code Matlab:
function yp=khidonghoc(t,y)
m = 10;
cd = 0.2;
g = 9.81;
w = m*g;
tspan=[0 5];
y0=[0;100;0;10];
yp = zeros(4,1);
yp(1)= y(2);
yp(2) = ((-cd/m)*y(2)*(y(2)^2+y(4)^2)^(0.5));
yp(3) = y(4);
yp(4) = ((-w/m)-(cd/m)*y(4)*(y(2)^2+y(4)^2)^(0.5));
[t,y]=ode45('khidonghoc',tspan,y0);
plot(t,y(:,1),y(:,3))
hold on;grid on;
xlabel('X-Displacement')
ylabel('Y-Displacement')
title('X vs Y Displacement')
end
Matlab error: "Not enough input arguments.
Error in khidonghoc (line 9)
yp(1)= y(2);"
Thanks you very much.

Best Answer

function main
  tspan=[0 5];
  y0=[0;100;0;10];
  [t,y]=ode45(@khidonghoc,tspan,y0);
  plot(t,y(:,1),t,y(:,3))
  hold on;grid on;
  xlabel('X-Displacement')
  ylabel('Y-Displacement')
  title('X vs Y Displacement')
end
function yp=khidonghoc(t,y)
  m = 10;
  cd = 0.2;
  g = 9.81;
  w = m*g;
  yp = zeros(4,1);
  yp(1)= y(2);
  yp(2) = ((-cd/m)*y(2)*(y(2)^2+y(4)^2)^(0.5));
  yp(3) = y(4);
  yp(4) = ((-w/m)-(cd/m)*y(4)*(y(2)^2+y(4)^2)^(0.5));
end
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