※ 引述《Rogii (Rogii)》之銘言：
: 如題:
: x(t) and x.................(1)
: v(x,t) ; v(x(t))=v(x)=v(t).........(2)
For example, forced harmonic motion
ma=-kx+F cost
a=(-k/m)x+(F/m)cost
that is a(x,t)
however, x should depend on t.
that is, a(x(t),t)
then da/dt=(-k/m)dx/dt-(F/m)sint
It is similar to v(x(t),t)
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