vpasolve() gives wrong solution

Question

John Arbolino 2020-9-13

0
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https://ww2.mathworks.cn/matlabcentral/answers/593062-vpasolve-gives-wrong-solution

评论： Walter Roberson 2020-9-17

I am trying to find the time at which my equation equals a value, but whenever I use vpasolve I am given a completely wrong answer of a negative time value. When I also add t>0 it gives no answer even though the function does equal that value at some positive t value.

syms s t
y(s) = 36/(s*(s^2+2*s+36));
response(t) = ilaplace(y(s))
s = vpasolve(response == 0.1)
st = vpasolve(response == 0.1, t>0)
response(s)

response(t) =

1 - exp(-t)*(cos(5.9161*t) + 0.1690*sin(5.9161*t))

s =

-226.9853

st =

Empty sym: 0-by-1

ans =

-1.0328e+62

You can see that when I try and use solve() without a specific time domain, I receieve a value S that when I plug back into response() is wildly off the value I am looking for. I know for a fact that the function response(t) does cross the value 0.1 in the positve time domain, I have graphed it.