STEP-BY-STEP ANSWER:
Step 1: Define the sales function S(t) where t represents time in years since the DVD release.
Step 2: Identify the differentiation rules applicable to the form of S(t) (e.g., power rule, product rule).
Step 3: Differentiate S(t) with respect to time t to obtain S'(t), which represents the rate of sales change.
Step 4: Interpret S'(t): a positive value indicates increasing sales, while a negative value indicates decreasing sales.
Final Answer: Differentiation provides S'(t), the instantaneous rate at which DVD sales are changing over the first 10 years.