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.

3 questions · Standard +0.5

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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.