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EXAMPLE 10Analyzing a Cost Function

The weekly cost C, in dollars, of manufacturing x lightbulbs is C(x)=7500+125x

  1. (a) Find the average rate of change of the weekly cost C of manufacturing from 100 to 101 lightbulbs.
  2. (b) Find the average rate of change of the weekly cost C of manufacturing from 1000 to 1001 lightbulbs.
  3. (c) Interpret the results from parts (a) and (b).

Solution(a) The weekly cost of manufacturing 100 lightbulbs is C(100)=7500+125100=7500+12,500$7611.80

The weekly cost of manufacturing 101 lightbulbs is C(101)=7500+125101=7500+12,625$7612.36

The average rate of change of the weekly cost C from 100 to 101 is ΔCΔx=C(101)C(100)1011007612.367611.801=$0.56

(b) The weekly cost of manufacturing 1000 lightbulbs is C(1000)=7500+1251000=7500+125,000$7853.55

The weekly cost of manufacturing 1001 lightbulbs is C(1001)=7500+1251001=7500+125,125$7853.73

The average rate of change of the weekly cost C from 1000 to 1001 is ΔCΔx=C(1001)C(1000)100110007853.737853.551=$0.18

(c) Part (a) tells us that the cost of manufacturing the 101st lightbulb is $0.56. From (b) we learn that the cost of manufacturing the 1001st lightbulb is only $0.18. The unit cost per lightbulb decreases as the number of lightbulbs manufactured per week increases.