Exercise 5.7.17 A retailer determines that their revenue R as a function of the amount a ( in thousands of dollars) spent on advertising can be approximated by 3(a) = 0-003q3 +1.35q2 +2q + 8000 0 g q 5 400 thousands of dollars. (a) Determine the concavity of R using intervals and determine if there are any inection points. (I?) If the retailer currently spends $140, 000 on advertising, should they consider increasing this amount