cinemática: tren de zero a b

d_{t}[x] = (a/b)·x(x+(-b))

( ( 1/(x+(-b)) )+(-1)·(1/x) )·d_{t}[x] = a

ln(x+(-b))+(-1)·ln(x) = at

(1+(-1)(b/x)) = e^{at}

(b/x)  = 1+(-1)·e^{at}

x(t) = ( b/(1+(-1)·e^{at}) )

d_{t}[x] = (ba)·( e^{at}/(1+(-1)·e^{at})^{2} )