When a person coughs, the trachea contracts, allowing air to be expelled at maximum velocity. It can be shown that during a cough the velocity of airflow is given by the fuction: v=kr^2(R-r) whereas r is the trachea's radius during a cough, R is the trachea's normal radius and k is the constant that depends on the length of the trachea. Find the rate at which velocity of expelled air is changing with respect to the trachea's radius during a cough. Is the velocity of airflow increasing or decreasing when r=1/2R? Why? Show all your work.