Modelling with Exponentials & Logarithms (AQA A Level Maths: Pure)

Write   in the form  .

Write   in the form  .
State whether this would represent exponential growth or exponential decay.

Write   in the form  .

Sketch the graph of .
State the coordinates of the -axis intercept.
Write down the equation of the asymptote.

By taking logarithms (base ) of both sides show that the equation

can be written as

Given    and  , find the values of  and .

By taking logarithms (base 10) of both sides show that the equation

can be written as

Given   and  , find the values of  and .

By taking logarithms (base 2) of both sides show that the equation

can be written as

Given   and  , find the values of and .

In an effort to prevent extinction scientists released some rare birds into a newly constructed nature reserve.

The population of birds, within the reserve, is modelled by

 is the number of birds after  years of being released into the reserve.

Write down the number of birds the scientists released into the nature reserve.

According to this model, how many birds will be in the reserve after 3 years?

How long will it take for the population of birds within the reserve to reach 500?

A simple model for the acceleration of a rocket, , is given as

where  is the time in seconds after lift-off.   is a constant.

What does the constant  represent?

After 10 seconds, the acceleration is .
Find the value of .

Find how long it takes for the acceleration of the rocket to reach 

Carbon-14 is a radioactive isotope of the element carbon.
Carbon-14 decays exponentially – as it decays it loses mass.
Carbon-14 is used in carbon dating to estimate the age of objects.

The time it takes the mass of carbon-14 to halve (called its half-life) is approximately 5700 years.

 A model for the mass of carbon-14, m g, in an object of age  years is

where and  are constants.

For an object initially containing 100g of carbon-14, write down the value of .

Briefly explain why, if ,  will equal g  when  years.

Using the values from part (b), show that the value of is to three significant figures.

A different object currently contains 60g of carbon-14.
In 2000 years’ time how much carbon-14 will remain in the object?

An exponential growth model for the number of bacteria in an experiment is of the form . is the number of bacteria and  is the time in hours since the experiment began.  and  are constants.  A scientist records the number of bacteria at various points over a six-hour period.  The results are shown in the table below.

, hours	0	2	4	6
, no. of bacteria	100	180	340	620
(3SF)	4.19	4.73	5.31	5.85

Plot the observations on the graph below - plotting against .

Using the points (0, 4.19) and (6, 5.85), find an equation for a line of best fit in the form , where and  are constants to be found.

The equation   can be written in the form  .
Use your answer to part (b) to estimate the values of , , and .

An exponential model of the form    is used to model the amount of a pain-relieving drug (D mg/ml) there is in a patient’s bloodstream,  hours after the drug was administered by injection.   and  are constants.

The graph below shows values of   plotted against  with a line of best fit drawn.

(i)        Use the graph and line of best fit to estimate at time .

(ii)       Work out the gradient of the line of best fit.

Use your answers to part (a) to write down an equation for the line of best fit in the form ,  where and  are constants.

Show that  can be rearranged to give 

Hence find estimates for the constants and .

Find the time when the amount of the pain-relieving drug in the patient’s bloodstream is 1.5 mg/ml.

A small company makes a profit of £2500 in its first year of business and £3700 in the second year.  The company decides they will use the model

to predict future years’ profits.

 is the profit in the  year of business.

 and  are constants.

Write down two equations connecting  and .

Find the values of and .

Find the predicted profit for years 3 and 4.

Show that

can be written in the form

Write   in the form  , giving the value of  to three significant figures.

Write   in the form  , giving the value of  to three significant figures.
State, and justify, whether this would represent exponential growth or decay.

Write   in the form  .

Sketch the graph of .
State the coordinates of the -axis intercept.
Write down the equation of the asymptote.

Show that the equation

can be written as

Rewrite the equation in the form  .

Show that the equation

can be written as

Rewrite the equation in the form  .

Show that the equation

can be written as

Rewrite the equation in the form  .

Scientists introduced a small number of rare breed deer to a large wildlife sanctuary.

The population of deer, within the sanctuary, is modelled by

 is the number of deer after  years of first being introduced to the sanctuary.

Write down the number of deer the scientists introduced to the sanctuary.

How many years does it take for the deer population to double?

Give one criticism of the model for population growth.

The scientists suggest that the population of deer are separated after either 25 years or when their population exceeds 400.
Find the earliest time the deer should be separated.

A simple model for the acceleration of a rocket, , is given as

where  is the time in seconds after lift-off.   is a constant.

After 4 seconds the acceleration of the rocket is .
Find the value of .

Find the time at which the acceleration of the rocket has increased by 200%.

Sketch the graph of the acceleration of the rocket, against time, stating the coordinates of the point that shows the initial acceleration of the rocket.

Carbon-14 is a radioactive isotope of the element carbon.
Carbon-14 decays exponentially – as it decays it loses mass.
Carbon-14 is used in carbon dating to estimate the age of objects.

The time it takes the mass of carbon-14 to halve (called its half-life) is approximately 5700 years

 A model for the mass of carbon-14,  g, in an object originally containing 100 g,
at time years is

where is a constant.

Find the value of , giving your answer to three significant figures.

The object is considered as having no radioactivity once the mass of carbon-14 it contains falls below 0.5 g. Find out how old the object would have to be, to be considered non-radioactive.

A different object currently contains 25g of carbon-14.
In 500 years’ time how much carbon-14 will remain in the object?

An exponential growth model for the number of bacteria in an experiment is of the form

 is the number of bacteria and  is the time in hours since the experiment began.  and  are constants.

 A scientist records the number of bacteria at various points over a six-hour period.
The results are in the table below.

, hours	0	2	4	6
, no. of bacteria	200	350	600	1100

Plot the observations on the graph below - plotting against .
Draw a line of best fit.

Find an equation for your line of best fit in the form .

Estimate the values of , , and .

An exponential model of the form

is used to model the amount of a pain-relieving drug (D mg/ml) there is in a patient’s bloodstream,  hours after the drug was administered by injection.   and  are constants.

The graph below shows values of plotted against 

Using the points marked and , find an equation for the line of best fit, giving your answer in the form , where and  are constants to be found.

Hence find estimates for the constants  and 

The patient is allowed a second injection of the drug once the amount of drug in the bloodstream falls below 1% of the initial dose.
Find, to the nearest minute, how long until the patient is allowed a second injection of the drug.

The annual profits, in thousands of pounds, of a small company in the first 4 years of business are given in the table below.

years in business	1	2	3	4
,annual profit	£3100	£4384	£5369	£6200

Using this data the company uses the model

to predict future years’ profits.   and  are constants.

Use data from the table to find the values of and .

Show that , where  and  take the values found in part (a).

State a potential problem with using the model to predict the profit in the company’s 12^th year of business.

Write   in the form  , giving the value of  to three significant figures.

Write    in the form  , giving the values of  and  to three significant
figures where necessary.

State, and justify, whether this would represent exponential growth or decay.

Write down the initial value of .

Sketch the graph of  .

State the coordinates of any points where the graph intercepts the coordinate axes.

Write down the equations of any asymptotes.

Rewrite the equation  in the form  .

Sketch the graph of by plotting    against .

Rewrite the equation    in the form 

Sketch the graph of against  .

Rewrite the equation   in the form   where  is an integer and  and  are rational numbers.

Sketch the graph of against .

Scientists introduced a small number of apes into a previously unpopulated forest.

The population of apes in the forest is modelled by

where  is the number of apes after  months of first being introduced to the forest.

State, with a reason, whether you would expect the value of  to be positive or negative.

After 8 months, the number of apes in the forest has increased by 50%.
Find the value of .

Scientists believe the forest cannot sustain a population of apes greater than 3000.
What length of time is the model for the population of the apes reliable for?

A manufacturer claims their flask will keep a hot drink warm for up to 7 hours.

In this sense, warm is considered to be  or higher.

Assuming a hot drink is made at and its temperature inside the flask is after exactly 7 hours, find:

a linear model for the temperature of the drink inside the flask of the form , and

an exponential model for the temperature of the drink inside the flask of the form

where  is the temperature of the drink in the flask after  hours and and  are constants.

Compare the rate of change of the temperature of the drink inside the flask of both models after 3 hours.

A user of the flask suggests that hot drinks are only kept warm for 5 hours.
Suggest a reason why the user’s experience may not be up to the claims of the manufacturer.

A simple model for the acceleration of a rocket, , is given as

where  is the time in seconds after lift-off.  and k are constants.

Negative time is often used in rocket launches as a way of counting down until lift off. Despite this the model above is still not suitable for use with negative  values
Briefly explain why.

After 5 seconds the acceleration of the rocket is and after 20 seconds its acceleration is .  Find the values of and .

A space enthusiast suggests that a linear model (of the form ) would be more suitable.
Using the figures in (b), explain why the enthusiast’s model is unrealistic.

Carbon-14 is a radioactive isotope of the element carbon.
Carbon-14 decays exponentially – as it decays it loses mass.
Carbon-14 is used in carbon dating to estimate the age of objects.

The time it takes carbon-14 to halve (called its half-life) is approximately 5700 years.

A model for the mass of carbon-14,  g, in an object, at time  years is

where and  are constants.

Briefly explain the meaning of the constant .

Find the value of , giving your answer in the form , where and  are integers to be found.

An object currently contains 200 g of carbon-14. In 20 000 years’ time, how much carbon-14, to the nearest gram, remains in the object?

The half-life of carbon-14 is believed to only be accurate to ±40 years.
A fossilised bone currently contains g of carbon-14.
It is estimated the bone would have originally contained  g of carbon-14.

Find upper and lower estimates for the age of the bone, giving your answers to two significant figures.

An exponential growth model for the number of bacteria in an experiment is of the form

 is the number of bacteria and  is the time in hours since the experiment began.  and   are constants.

A scientist records the number of bacteria at various points over a six-hour period.
The results are in the table below.

By plotting  against , drawing a line of best fit and finding its equation, estimate the values of , , and .

What does the model predict for the value of after twelve hours?
Comment on the reliability of this prediction.

An exponential model of the form

is used to model the concentration of a pain-relieving drug (D mg/ml) in a patient’s bloodstream  hours after the drug was administered by injection.   and  are constants.

The graph below shows values of plotted against 

Find estimates for the constants and .

Find the time, to the nearest minute, at which the rate of decrease of the concentration of the drug in the patient’s bloodstream is 12 mg/ml/hour.

The annual profits, in thousands of pounds, of a small company in the first 4 years of business are given in the table below.

, years in business	1	2	3	4
( is annual profit)	3.74	3.86	3.94	4.01

Using this data the company uses the model

to predict future years’ profits.   and  are constants.

Use the results in the table to estimate the values of and .

Many new companies make a loss in their first year of business.
Briefly explain why, in such circumstances, a model of the form used above would not be suitable.