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import sympy as sm
a, l, t = sm.symbols('a l t')
phi = sm.Function('phi')(t)
dt = sm.Matrix([l*sm.sin(phi), l*sm.cos(phi) + 3*t**2, 0]).diff(t)
dt

Funktionierte bei mir problemlos.
Vielleicht hilft phi = sp.Function(‚phi‘)(t)
NB: Besser Du schreibst auf englisch, das können mehr Leute lesen.
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