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Simplifying solution of a differential equation

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Aleem Andrew
Aleem Andrew el 14 de En. de 2021
Comentada: Aleem Andrew el 14 de En. de 2021
The most simplified version of ySol(t), the solution to the differential equation below, is 1.5*sin(2t+0.7297), but the output of the following code is in terms of exponential functions. Can someone explain how the output can be further simplified?
syms y(t) m k
Dy = diff(y,t); Dy2 = diff(y,t,2);
ode = m*Dy2 + k*y == 0;
cond = [y(0) == 1,Dy(0) == sqrt(5)];
ySol(t) = dsolve(ode,cond)
ySol(t) = simplify(ySol(t),'steps',500)
pretty(ySol(t))
  4 comentarios
Walter Roberson
Walter Roberson el 14 de En. de 2021
When m and k are symbolic, you get symbolic expressions for the coefficients, not numeric ones like you show as your desired output.
Aleem Andrew
Aleem Andrew el 14 de En. de 2021
That is because there is an additional equation relating k and m, sqrt(k/m) = 2, that I tried to include in the dsolve command to solve the system but got an error message when trying to solve a system of equations, [ode sqrt(k/m) == 2]. Instead the ode = m*Dy2 + k*y == 0; line can be modified to ode = (k/4)*Dy2 + k*y == 0; to obtain the numeric solution.

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Respuesta aceptada

Walter Roberson
Walter Roberson el 14 de En. de 2021
m = rand(); k = rand();
syms y(t)
Dy = diff(y,t);
Dy2 = diff(y,t,2);
ode = m*Dy2 + k*y == 0;
cond = [y(0) == 1,Dy(0) == sqrt(5)];
ySol(t) = dsolve(ode,cond)
ySol(t) = 
ySol(t) = simplify(ySol(t),'steps',500)
ySol(t) = 
pretty(ySol(t))
/ sqrt(43198488722811199054095930230) t \ sqrt(8639697744562239810819186046) sin| ------------------------------------- | 5 / sqrt(43198488722811199054095930230) t \ \ 157178273090335 / cos| ------------------------------------- | + --------------------------------------------------------------------------------- \ 157178273090335 / 274837532398538
vpa(ySol(t), 5)
ans = 

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