Final answer:
To model the given situation, write the differential equation dr/dt = -k/r.
Step-by-step explanation:
To write a differential equation that models the situation described, we can use the proportionality constant k. The rate of change of the radius r of a tree trunk is inversely proportional to the radius. In mathematical terms, we can write this as:
dr/dt = -k/r
In this equation, dr/dt represents the rate of change of the radius with respect to time, k is the proportionality constant, and r is the radius of the tree trunk.