cinematica: tren de (-b) a b

d_{t}[x] = (a/(2b))·( x^{2}+(-1)·b^{2} )

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

ln(x+(-b))+(-1)·ln(x+b) = at

1+(-1)·( (2b)/(x+b) ) = e^{at}

( (2b)/(x+b) ) = 1+(-1)·e^{at}

x(t) = ( (2b)/(1+(-1)·e^{at}) )+(-b)

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