Final answer:
The Mean Value Theorem is commonly used to find the average value of a function over a closed interval.
Step-by-step explanation:
The theorem commonly used to find the average value of a function over a closed interval is the Mean Value Theorem. The Mean Value Theorem states that if a function is continuous on a closed interval and differentiable on the open interval, then at some point within the open interval, the instantaneous rate of change (the derivative) is equal to the average rate of change (the slope of the secant line connecting the endpoints).