Natural resource depletion. The instantaneous rate of change in demand for U.S. lumber since 1970 (t= 0). in billions of cubic feet per year, is given by
Q'(t)=12 + 0.006t2 0 ≤ t ≤ 50
Find the area between the graph of Q' and the t axis over the interval [15,20], and interpret the results.