i need solve md2x/dt2 + cdx/dt + k*x= 0 with matlab and I have encountered an error Undefined function or variable 't'. please help me

Question

Thuan Huynh 2019-4-22

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https://ww2.mathworks.cn/matlabcentral/answers/457829-i-need-solve-m-d2x-dt2-c-dx-dt-k-x-0-with-matlab-and-i-have-encountered-an-error-undefined-func

编辑： James Tursa 2019-4-22

function dx = kc(t,x)
global cm km
dx = [-cm*x(1) - km*x(2);x(1)];
tspan = [0 15]; 
ic = [0 1]; 
global cm km
m = 20; c(1)=5; c(2)= 40; c(3) = 200; k = 20; 
 km=k/m; 
 for n = 1:3 
cm = c(n)/m;  
[t,x] = ode45('kc',tspan,ic); 
plot(t,x(:,2)); grid; 
xlabel('time - sec.'); 
ylabel ('displacement-m'); 
title('m*(d2x/dt2)+c*(dx/dt)+k*x=0; m=10 kg, k=40 N/m') 
hold on 
end

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Answer 1

James Tursa 2019-4-22

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https://ww2.mathworks.cn/matlabcentral/answers/457829-i-need-solve-m-d2x-dt2-c-dx-dt-k-x-0-with-matlab-and-i-have-encountered-an-error-undefined-func#answer_371705

编辑：James Tursa 2019-4-22

Is this code all together in one file as written above? You need to have your driver code (the code that calls ode45) first, and your derivative function code last or in another file. The ode45 function call should not be inside your derivative function.