Applied rate of change

3 The length, \(x\) metres, of a Green Anaconda snake which is \(t\) years old is given approximately by the formula $$x = 0.7 \sqrt { } ( 2 t - 1 ) ,$$ where \(1 \leqslant t \leqslant 10\). Using this formula, find

1. \(\frac { \mathrm { d } x } { \mathrm {~d} t }\),
2. the rate of growth of a Green Anaconda snake which is 5 years old.

13. Figure 5 A colony of ants is being studied. The number of ants in the colony is modelled by the equation $$P = 200 - \frac { 160 \mathrm { e } ^ { 0.6 t } } { 15 + \mathrm { e } ^ { 0.8 t } } \quad t \in \mathbb { R } , t \geqslant 0$$ where \(P\) is the number of ants, measured in thousands, \(t\) years after the study started. A sketch of the graph of \(P\) against \(t\) is shown in Figure 5

1. Calculate the number of ants in the colony at the start of the study.
2. Find \(\frac { \mathrm { d } P } { \mathrm {~d} t }\) The population of ants initially decreases, reaching a minimum value after \(T\) years, as shown in Figure 5
3. Using your answer to part (b), calculate the value of \(T\) to 2 decimal places.
   (Solutions based entirely on graphical or numerical methods are not acceptable.)

9. A rare species of mammal is being studied. The population \(P\), \(t\) years after the study started, is modelled by the formula $$P = \frac { 900 \mathrm { e } ^ { \frac { 1 } { 4 } t } } { 3 \mathrm { e } ^ { \frac { 1 } { 4 } t } - 1 } , \quad t \in \mathbb { R } , \quad t \geqslant 0$$ Using the model,

1. calculate the number of mammals at the start of the study,
2. calculate the exact value of \(t\) when \(P = 315\) Give your answer in the form \(a \ln k\), where \(a\) and \(k\) are integers to be determined.
3. 1. Find \(\frac { \mathrm { d } P } { \mathrm {~d} t }\)
   2. Hence find the value of \(\frac { \mathrm { d } P } { \mathrm {~d} t }\) when \(t = 8\), giving your answer to 2 decimal places.

8. \begin{figure}[h] \end{figure} The number of rabbits on an island is modelled by the equation $$P = \frac { 100 \mathrm { e } ^ { - 0.1 t } } { 1 + 3 \mathrm { e } ^ { - 0.9 t } } + 40 , \quad t \in \mathbb { R } , t \geqslant 0$$ where \(P\) is the number of rabbits, \(t\) years after they were introduced onto the island.
A sketch of the graph of \(P\) against \(t\) is shown in Figure 3.

1. Calculate the number of rabbits that were introduced onto the island.
2. Find \(\frac { \mathrm { d } P } { \mathrm {~d} t }\) The number of rabbits initially increases, reaching a maximum value \(P _ { T }\) when \(t = T\)
3. Using your answer from part (b), calculate
   1. the value of \(T\) to 2 decimal places,
   2. the value of \(P _ { T }\) to the nearest integer.
      (Solutions based entirely on graphical or numerical methods are not acceptable.) For \(t > T\), the number of rabbits decreases, as shown in Figure 3, but never falls below \(k\), where \(k\) is a positive constant.
4. Use the model to state the maximum value of \(k\).

4 The temperature \(T ^ { \circ } \mathrm { C }\) of a liquid at time \(t\) minutes is given by the equation $$T = 30 + 20 \mathrm { e } ^ { - 0.05 t } , \quad \text { for } t \geqslant 0 .$$ 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 \(40 ^ { \circ } \mathrm { C }\).

5 At time \(t\) minutes after an oven is switched on, its temperature \(\theta ^ { \circ } \mathrm { C }\) is given by $$\theta = 200 - 180 \mathrm { e } ^ { - 0.1 t }$$

1. State the value which the oven's temperature approaches after a long time.
2. Find the time taken for the oven's temperature to reach \(150 ^ { \circ } \mathrm { C }\).
3. Find the rate at which the temperature is increasing at the instant when the temperature reaches \(150 ^ { \circ } \mathrm { C }\).

11 A ball moves in a straight line and passes through two fixed points, \(A\) and \(B\), which are 0.5 m apart. The ball is moving with a constant acceleration of \(0.39 \mathrm {~m} \mathrm {~s} ^ { - 2 }\) in the direction \(A B\).
The speed of the ball at \(A\) is \(1.9 \mathrm {~ms} ^ { - 1 }\)
Find the speed of the ball at \(B\).
Circle your answer.
\(2 \mathrm {~m} \mathrm {~s} ^ { - 1 }\)
\(3.2 \mathrm {~m} \mathrm {~s} ^ { - 1 }\)
\(3.8 \mathrm {~m} \mathrm {~s} ^ { - 1 }\)
\(4 \mathrm {~m} \mathrm {~s} ^ { - 1 }\) A particle \(P\), of mass \(m\) kilograms, is attached to one end of a light inextensible string.
The other end of this string is held at a fixed position, \(O\).
\(P\) hangs freely, in equilibrium, vertically below \(O\).
Identify the statement below that correctly describes the tension, \(T\) newtons, in the string as \(m\) varies. Tick \(( \checkmark )\) one box.
\(T\) varies along the string, with its greatest value at \(O\) □
\(T\) varies along the string, with its greatest value at \(P\) □
\(T = 0\) because the system is in equilibrium □
\(T\) is directly proportional to \(m\) □
\includegraphics[max width=\textwidth, alt={}, center]{9f84ae5b-15d9-40e7-bdc2-a7a8715082b4-15_2488_1716_219_153}

11 A particle's displacement, \(r\) metres, with respect to time, \(t\) seconds, is defined by the equation $$r = 3 \mathrm { e } ^ { 0.5 t }$$ Find an expression for the velocity, \(v \mathrm {~m} \mathrm {~s} ^ { - 1 }\), of the particle at time \(t\) seconds.
Circle your answer.
\(v = 1.5 \mathrm { e } ^ { 0.5 t }\)
\(v = 6 \mathrm { e } ^ { 0.5 t }\)
\(v = 1.5 t \mathrm { e } ^ { 0.5 t }\)
\(v = 6 t e ^ { 0.5 t }\)