## Functions Modeling Change: A Preparation for Calculus, 5th Edition

Oftentimes, functions modeling real-world situations involve certain restraints. Domain restriction: If W = f(x) shows W pounds lost daily from $x$ daily hours of working out, $x$ must be a positive value (you cannot work out for a negative amount of time). Range restriction: If P = f(y) shows the population P of planet earth at the beginning of the year $y$ years after the year 2000, $P$ must be positive (population cannot be negative).