Solución

a.   $v\left( t \right) = {1 \over {1 + {t^2}}}$

b.   $$$a\left( t \right) = {{ - 2t} \over {{{\left( {1 + {t^2}} \right)}^2}}}$$$

c.   (a) $0.2$, $0.06$, $0.03$   (b) $-0.16$, $-0.028$, $-0.0088$

d.   El disco de hockey está desacelerando a los 2, 4 y 6 segundos