4. [4 marks] The apparent temperature of the air at a certain temperature and humidity level is known as the heat index. If :1: represents the relative humidity (in percent), then the heat index (in degrees Fahrenheit) for an air temperature of IOODF can be approximated by the function ' ) x) = 0.009532 + 0.1393: + 91.875. (a) When the humidity level increases from 30% to 50%, What is the average rate of change of the heat index? Interpret your answer. (b) At what rate does the heat index change when the humidity level reaches 50%? Interpret your answer. k