Calculus with exponential models

Use differentiation to find rates of change (dy/dx or dx/dy) for exponential or logarithmic functions, often in modelling contexts.

9 questions · Moderate -0.1

1.06i Exponential growth/decay: in modelling context
Sort by: Default | Easiest first | Hardest first
Edexcel P3 2021 June Q5
7 marks Standard +0.3
5. \begin{figure}[h]
\includegraphics[alt={},max width=\textwidth]{76205772-5395-4ab2-96f9-ad9803b8388c-16_582_737_248_607} \captionsetup{labelformat=empty} \caption{Figure 1}
\end{figure} The growth of duckweed on a pond is being studied. The surface area of the pond covered by duckweed, \(A \mathrm {~m} ^ { 2 }\), at a time \(t\) days after the start of the study is modelled by the equation $$A = p q ^ { t } \quad \text { where } p \text { and } q \text { are positive constants }$$ Figure 1 shows the linear relationship between \(\log _ { 10 } A\) and \(t\).
The points \(( 0,0.32 )\) and \(( 8,0.56 )\) lie on the line as shown.
  1. Find, to 3 decimal places, the value of \(p\) and the value of \(q\). Using the model with the values of \(p\) and \(q\) found in part (a),
  2. find the rate of increase of the surface area of the pond covered by duckweed, in \(\mathrm { m } ^ { 2 }\) / day, exactly 6 days after the start of the study.
    Give your answer to 2 decimal places. \includegraphics[max width=\textwidth, alt={}, center]{76205772-5395-4ab2-96f9-ad9803b8388c-19_2649_1840_117_114}
OCR H240/01 2018 June Q11
11 marks Standard +0.8
11 In a science experiment a substance is decaying exponentially. Its mass, \(M\) grams, at time \(t\) minutes is given by \(M = 300 e ^ { - 0.05 t }\).
  1. Find the time taken for the mass to decrease to half of its original value. A second substance is also decaying exponentially. Initially its mass was 400 grams and, after 10 minutes, its mass was 320 grams.
  2. Find the time at which both substances are decaying at the same rate.
OCR MEI Paper 1 2023 June Q11
10 marks Standard +0.3
11 The height \(h \mathrm {~cm}\) of a sunflower plant \(t\) days after planting the seed is modelled by \(\mathrm { h } = \mathrm { a } + \mathrm { b }\) Int for \(t \geqslant 9\), where \(a\) and \(b\) are constants. The sunflower is 10 cm tall 10 days after planting and 200 cm tall 85 days after planting.
    1. Show that the value of \(b\) which best models these values is 88.8 correct to \(\mathbf { 3 }\) significant figures.
    2. Find the corresponding value of \(a\).
    1. Explain why the model is not suitable for small positive values of \(t\).
    2. Explain why the model is not suitable for very large positive values of \(t\).
  1. Show that the model indicates that the sunflower grows to 1 m in height in less than half the time it takes to grow to 2 m .
  2. Find the value of \(t\) for which the rate of growth is 3 cm per day.
OCR C3 Q3
6 marks Moderate -0.3
The mass, \(m\) grams, of a substance at time \(t\) years is given by the formula $$m = 180e^{-0.017t}.$$
  1. Find the value of \(t\) for which the mass is 25 grams. [3]
  2. Find the rate at which the mass is decreasing when \(t = 55\). [3]
OCR C3 Q6
9 marks Moderate -0.3
  1. \(t\)01020
    \(X\)275440
    The quantity \(X\) is increasing exponentially with respect to time \(t\). The table above shows values of \(X\) for different values of \(t\). Find the value of \(X\) when \(t = 20\). [3]
  2. The quantity \(Y\) is decreasing exponentially with respect to time \(t\) where $$Y = 80e^{-0.02t}.$$
    1. Find the value of \(t\) for which \(Y = 20\), giving your answer correct to 2 significant figures. [3]
    2. Find by differentiation the rate at which \(Y\) is decreasing when \(t = 30\), giving your answer correct to 2 significant figures. [3]
OCR C3 Q5
7 marks Moderate -0.3
A substance is decaying in such a way that its mass, \(m\) kg, at a time \(t\) years from now is given by the formula $$m = 240e^{-0.04t}.$$
  1. Find the time taken for the substance to halve its mass. [3]
  2. Find the value of \(t\) for which the mass is decreasing at a rate of 2.1 kg per year. [4]
OCR C3 2013 January Q4
6 marks Moderate -0.3
The mass, \(m\) grams, of a substance is increasing exponentially so that the mass at time \(t\) hours is given by $$m = 250e^{0.02t}.$$
  1. Find the time taken for the mass to increase to twice its initial value, and deduce the time taken for the mass to increase to 8 times its initial value. [3]
  2. Find the rate at which the mass is increasing at the instant when the mass is 400 grams. [3]
OCR MEI C3 Q4
Moderate -0.8
The temperature \(T°C\) of a liquid at time \(t\) minutes is given by the equation $$T = 30 + 20e^{-0.05t}, \quad \text{for } t \geq 0.$$ Write down the initial temperature of the liquid, and find the initial rate of change of temperature. Find the time at which the temperature is \(40°C\).
SPS SPS SM Mechanics 2022 February Q6
9 marks Moderate -0.3
A scientist is studying the growth of two different populations of bacteria. The number of bacteria, \(N\), in the first population is modelled by the equation $$N = Ae^{kt} \quad t \geq 0$$ where \(A\) and \(k\) are positive constants and \(t\) is the time in hours from the start of the study. Given that • there were 1000 bacteria in this population at the start of the study • it took exactly 5 hours from the start of the study for this population to double
  1. find a complete equation for the model. [4]
  2. Hence find the rate of increase in the number of bacteria in this population exactly 8 hours from the start of the study. Give your answer to 2 significant figures. [2]
The number of bacteria, \(M\), in the second population is modelled by the equation $$M = 500e^{1.4t} \quad t \geq 0$$ where \(k\) has the value found in part (a) and \(t\) is the time in hours from the start of the study. Given that \(T\) hours after the start of the study, the number of bacteria in the two different populations was the same,
  1. find the value of \(T\). [3]