The concentration of a dye in a chemical solution is modeled by \(c(t)=\frac{10t}{(t+4)^2}\), where \(c(t)\) is measured in grams per liter and \(t\ge 0\) is the time in hours after the solution is prepared.
a) Explain what \(c'(t)\) means in this context and give its units.
b) Find the time when the dye concentration is greatest. State when the concentration is increasing and when it is decreasing.
c) Find and interpret the instantaneous rates of change at \(t=0\), \(t=4\), and \(t=8\).
d) Find the average rate of change of the concentration over \([0,8]\). Compare it with the instantaneous rate of change at \(t=4\), and explain the difference.
Hints
- What does the slope of a function represent in a real-world context?
- How can a derivative be used to locate a maximum?
- What is the difference between a slope at one instant and an average slope over an interval?
- What signs should the derivative have when a function is increasing or decreasing?
Solution
1. The derivative \(c'(t)\) gives the instantaneous rate of change of dye concentration. Its units are \(\frac{\text{g}}{\text{L}\cdot\text{h}}\).
2. Differentiate using the quotient rule: \(c'(t)=\frac{10(t+4)^2-20t(t+4)}{(t+4)^4}=\frac{40-10t}{(t+4)^3}\).
3. Set the derivative equal to zero: \(40-10t=0\), so \(t=4\). Because \(c'(t)>0\) for \(0\le t<4\) and \(c'(t)<0\) for \(t>4\), the concentration is greatest at \(t=4\,\text{h}\). It increases on \([0,4)\) and decreases on \((4,\infty)\).
4. Evaluate the derivative: \(c'(0)=\frac{5}{8}=0.625\), \(c'(4)=0\), and \(c'(8)=-\frac{5}{216}\approx -0.0231\), all in \(\frac{\text{g}}{\text{L}\cdot\text{h}}\). Thus the concentration is initially increasing, is momentarily unchanged at its maximum, and is decreasing at \(t=8\).
5. The average rate of change is \(\frac{c(8)-c(0)}{8-0}=\frac{5}{72}\approx 0.0694\,\frac{\text{g}}{\text{L}\cdot\text{h}}\).
6. This average is positive because the concentration at \(t=8\) is greater than at \(t=0\), even though the instantaneous rate at \(t=4\) is zero because the concentration has a maximum there.
Answer
a) \(c'(t)\) is the instantaneous rate of change of concentration, measured in \(\frac{\text{g}}{\text{L}\cdot\text{h}}\).
b) The maximum occurs at \(t=4\,\text{h}\). The concentration increases on \([0,4)\) and decreases on \((4,\infty)\).
c) \(c'(0)=0.625\,\frac{\text{g}}{\text{L}\cdot\text{h}}\), \(c'(4)=0\,\frac{\text{g}}{\text{L}\cdot\text{h}}\), and \(c'(8)=-\frac{5}{216}\,\frac{\text{g}}{\text{L}\cdot\text{h}}\approx -0.0231\,\frac{\text{g}}{\text{L}\cdot\text{h}}\).
d) The average rate is \(\frac{5}{72}\,\frac{\text{g}}{\text{L}\cdot\text{h}}\approx 0.0694\,\frac{\text{g}}{\text{L}\cdot\text{h}}\). It is positive, while \(c'(4)=0\), because \(t=4\) is the instant when the concentration reaches its maximum.