Trachea Contraction Coughing forces the trachea (windpipe) to contract, which affects the velocity $v$ of the air passing through the trachea. The velocity of the air during
coughing is
$$v=k(R-r) r^{2}, \quad 0 \leq r<R$$
where $k$ is a constant, $R$ is the normal radius of the trachea, and $r$ is the radius during coughing. What radius will produce the maximum air velocity?