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以下Mathematica代码生成高度振荡的图.我只想绘制情节的下部包络但不知道如何.任何建议都值得赞赏.

tk0 = \[Theta]'[t]*\[Theta]'[t] - \[Theta][t]*\[Theta]''[t]
tk1 = \[Theta]''[t]*\[Theta]''[t] - \[Theta]'[t]*\[Theta]'''[t]
a = tk0/Sqrt[tk1]
f = Sqrt[tk1/tk0]
s =
 NDSolve[{\[Theta]''[t] + \[Theta][t] - 0.167 \[Theta][t]^3 == 
    0.005 Cos[t - 0.5*0.00009*t^2], \[Theta][0] == 0, \[Theta]'[0] == 
    0}, \[Theta], {t, 0, 1000}]

Plot[Evaluate  [f /. s], {t, 0, 1000}, 
 Frame -> {True, True, False, False}, 
 FrameLabel -> {"t", "Frequency"}, 
 FrameStyle -> Directive[FontSize -> 15], Axes -> False]