This document discusses differentiation and finding the gradient function or derivative of functions. It begins by explaining how to find the derivative of a function numerically by approximating the gradient between two nearby points and then taking the limit as the distance between the points approaches zero. This leads to a general formula for finding the derivative using limits. Examples are then provided of finding the derivatives of polynomial functions like x^2 and x^4 using this first principles approach. The document then introduces a quicker rule for differentiating terms of the form x^n, where the derivative is nx^{n-1}. Several examples are given of applying this rule.
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1. P1 Chapter 12 :: Differentiation
jfrost@tiffin.kingston.sch.uk
www.drfrostmaths.com
@DrFrostMaths
Last modified: 14th October 2020
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3. Chapter Overview
1:: Find the derivative of
polynomials.
4:: Find and understand the second
derivative
π2π¦
ππ₯2 or πβ²β² π₯
If π¦ = π₯4
β 3π₯2
determine
π2π¦
ππ₯2
2:: Find equations of tangents and normal
to curves.
Those who have done either IGCSE Mathematics, IGCSE Further Mathematics or Additional
Mathematics would have encountered this content. Otherwise it will be completely new!
The point π 3,9 lies on the curve πΆ with
equation π¦ = π₯2. Determine the equation
of the tangent to πΆ at the point π.
3:: Identify increasing and
decreasing functions.
Find the range of values for
which π π₯ = π₯3
β π₯ is
increasing.
5:: Find stationary points and
determine their nature.
Find the stationary points of
π¦ = π₯3 β π₯ and state whether each
is a maximum or minimum point.
If π¦ = 3π₯2 + π₯, find
ππ¦
ππ₯
6:: Sketch a gradient function.
Draw π¦ = π₯3 and its gradient
function on the same axes.
7:: Model real-life problems.
4. Gradient Function
For a straight line, the
gradient is constant:
π = 3
However, for a curve the
gradient varies. We can no
longer have a single value
for the gradient; we ideally
want an expression in terms
of π that gives us the
gradient for any value of π₯
(unsurprisingly known as the
gradient function).
π -3 -2 -1 0 1 2 3
Gradient -6 -4 -2 0 2 4 6
At GCSE, you found
the gradient of a
curve at a particular
point by drawing a
tangent.
π¦ = π₯2
? ? ? ? ? ? ?
π = β6
π = β4
π = β2
π = 0
π = 2
π = 4
π = 6
By looking at the relationship between π₯ and the
gradient at that point, can you come up with an
expression, in terms of π₯ for the gradient?
πΊπππππππ‘ πΉπ’πππ‘πππ = ππ
?
5. Finding the Gradient Function
The question is then: Is there a method to work out the gradient function
without having to draw lots of tangents and hoping that we can spot the rule?
5,25
1
?
π¦ = π₯2
6,36
11
?
π =
Ξπ¦
Ξπ₯
=
11
1
= 11
?
To approximate the gradient on the curve
π¦ = π₯2
when π₯ = 5, we could pick a point
on the curve just slightly to the right, then
find the gradient between the two points:
As the second point gets closer and closer,
the gradient becomes a better
approximation of the true gradient:
5,25
0.01
5.01,25.1001
0.1001
?
π =
0.1001
0.01
= 10.01
? The actual gradient
when π₯ = 5 is 10, so
this approximation is
damn close!
6. Finding the Gradient Function
This gives us a numerical method to get the gradient at a particular π, but doesnβt
give us the gradient function in general. Letβs use exactly the same method, but keep
π₯ general, and make the βsmall changeβ (which was previously 0.01) βββ:
ππππππππ‘ = lim
ββ0
π₯ + β 2 β π₯2
β
= lim
ββ0
2π₯β + β2
β
= lim
ββ0
2π₯ + β
= 2π₯
π₯, π₯2
π₯ + β, π₯ + β 2
As always, gradient is
change in π¦ over change
in π₯.
The β disappears as β
tends towards 0, i.e. we
can effectively treat it
as 0 at this point.
?
?
?
?
The lim means βthe limit of
the following expression as β
tends towards 0β.
For example, lim
π₯ββ
1
π₯
= 0,
because as π₯ βtends towardsβ
infinity, the βlimitingβ value of
the expression is 0.
And voila, we got the
2π₯ we saw earlier!
?
?
?
?
π = ππ
7. Finding the Gradient Function
! The gradient function, or derivative, of the curve π¦ = π(π₯) is
written as πβ²(π₯) or
ππ¦
ππ₯
.
πβ²
π₯ = lim
ββ0
π π₯ + β β π π₯
β
The gradient function can be used to find the gradient of the
curve for any value of π₯.
We will soon see an easier/quicker way to
differentiate expressions like π¦ = π₯3
β π₯ without
using βlimitsβ. But this method, known as
differentiating by first principles, is now in the A
Level syllabus, and you could be tested on it!
Notation Note:
Whether we use
ππ¦
ππ₯
or πβ²(π₯) for the
gradient function depends on whether we
use π¦ = or π π₯ = to start with:
π¦ = π₯2 β
ππ¦
ππ₯
= 2π₯
π π₯ = π₯2 β πβ² π₯ = 2π₯
βLagrangeβs
notationβ
Thereβs in fact a third way to indicate the gradient function,
notation used by Newton: (but not used at A Level)
π¦ = π₯2
β π¦ = 2π₯
Advanced Notation Note:
Rather than β for the small change in π₯, the
formal notation is πΏπ₯. So actually:
πβ²
π₯ = lim
πΏπ₯β0
π π₯ + πΏπ₯ β π π₯
πΏπ₯
So we in fact have 3 symbols for βchange inβ!
β’ Ξπ₯: any change in π₯ (as seen in Chp5: π =
Ξπ¦
Ξπ₯
)
β’ πΏπ₯: a small change in π₯
β’ ππ₯: an infinitesimally small change in π₯
So the estimated gradient using some point close by was
πΏπ¦
πΏπ₯
, but in the βlimitβ as πΏπ₯ β 0,
πΏπ¦
πΏπ₯
β
ππ¦
ππ₯
βLeibnizβs notationβ
8. Example
!
The point π΄ with coordinates 4,16 lies on the curve with equation π¦ = π₯2.
At point π΄ the curve has gradient π.
a) Show that π = lim
ββ0
8 + β
b) Deduce the value of π.
π = lim
ββ0
π 4 + β β π 4
β
= lim
ββ0
4 + β 2 β 42
β
= lim
ββ0
16 + 8β + β2 β 16
β
= lim
ββ0
8β + β2
β
= lim
ββ0
8 + β
π = 8
As β β 0, clearly the limiting
value of 8 + β is 8.
Function is π π₯ = π₯2
Use the βdifferentiation by first
principlesβ formula.
a
b
?
?
9. Test Your Understanding
Prove from first principles that the derivative of π₯4 is 4π₯3.
πβ² π₯ = lim
ββ0
π π₯ + β β π π₯
β
= lim
ββ0
π₯ + β 4 β π₯4
β
= lim
ββ0
π₯4
+ 4π₯3
β + 6π₯2
β2
+ 4π₯β3
+ β4
β π₯4
β
= lim
ββ0
4π₯3
β + 6π₯2
β2
+ 4π₯β3
+ β4
β
= lim
ββ0
4π₯3
+ 6π₯2
β + 4π₯β2
+ β3
= 4π₯3
As β β 0, any term involving a
multiplication by β will become 0.
?
Helping Hand:
βRow 4β of Pascalβs
Triangle is:
1 4 6 4 1
Youβre welcome.
10. Exercise 12B
Pearson Pure Mathematics Year 1/AS
Pages 261-262
(Note that Exercise 12A was skipped in these slides)
Donβt forget the βChallengeβ question on Page 262!
Reminder:
πβ²(π₯) and
ππ¦
ππ₯
both mean the gradient function,
also known as the derivative of π¦.
πβ² π₯ =
ππ¦
ππ₯
= lim
ββ0
π π₯ + β β π π₯
β
11. ME-WOW!
Just for your interestβ¦
Why couldnβt we just immediately make
β equal to 0 in lim
ββ0
π₯+β 2βπ₯2
β
?
God, not another
one of theseβ¦
If we just stick β = 0 in straight away:
lim
ββ0
π₯ + 0 2
β π₯2
0
= lim
ββ0
0
0
Wait, uh ohβ¦
0
0
is known as an indeterminate form.
Whereas we know what happens with
expressions like
1
0
(i.e. its value tends towards
infinity), indeterminate forms are bad because
their values are ambiguous, and prevent πππ
expressions from being evaluated.
Consider
0
0
for example: 0 divided by anything
usually gives 0, but anything divided by 0 is
usually infinity. We can see these conflict.
Can you guess some other indeterminate forms, i.e.
expressions whose value is ambiguous?
π
π
β
β
π Γ β β β β ππ
πβ
βπ
Thankfully, when indeterminate forms appear in πππ
expressions, there are variety of techniques to turn the
expression into one that doesnβt have any indeterminate forms.
One simple technique, that worked in our example, is to
expand and simplify. This gave us lim
ββ0
2π₯ + β and 2π₯ + 0
clearly is fine. Other techniques are more advanced.
This is important when sketching harder functions:
http://www.drfrostmaths.com/resources/resource.php?rid=163
?
12. Differentiating π₯π
Thankfully, thereβs a quick way to differentiate terms of the form π₯π
(where π is a constant) with having to use first principles every time:
! If π¦ = ππ₯π
then
ππ¦
ππ₯
= πππ₯πβ1
(where π, π are constants)
i.e. multiply by the power and reduce the power by 1
Examples:
π¦ = π₯5
β
ππ¦
ππ₯
= 5π₯4 Power is 5, so multiply by 5 then
reduce power by 5.
π(π₯) = π₯
1
2 β πβ² π₯ =
1
2
π₯β
1
2
The power need not be an integer!
Remember to use πβ²
π₯ not
ππ¦
ππ₯
π¦ = 2π₯6 β
ππ¦
ππ₯
= 12π₯5
π π₯ =
π₯
π₯4
= π₯β3 β πβ² π₯ = β3π₯β4
Why would it be incorrect to say that π = ππ
differentiates to
π π
π π
= π ππβπ
?
The rule only works when the base is π₯ and the
power is a constant. Neither is true here! Note
that π₯π
is βa power of π₯β whereas 2π₯
is an
exponential term (which you will encounter
more in Chp14), and therefore differentiate
differently. You will learn how to differentiate
exponential terms in Year 2.
?
?
?
? ?
π¦ = π₯6 = π₯3 β
ππ¦
ππ₯
= 3π₯2
? ?
?
14. Differentiating Multiple Terms
Differentiate π¦ = π₯2 + 4π₯ + 3
First thing to note:
If π¦ = π π₯ + π π₯ then
ππ¦
ππ₯
= πβ²
π₯ + πβ²
π₯
i.e. differentiate each
term individually in a
sum/subtraction.
ππ¦
ππ₯
= 2π₯ + 4
? ? ?
π¦ = 4π₯ = 4π₯1
Therefore applying the usual rule:
ππ¦
ππ₯
= 4π₯0
= 4
Alternatively, if you compare π¦ = 4π₯ to
π¦ = ππ₯ + π, itβs clear that the gradient is
fixed and π = 4.
π¦ = 3 = 3π₯0
Therefore applying the usual rule:
ππ¦
ππ₯
= 0π₯β1
= 0
Alternatively, if you sketch π¦ = 4, the line is
horizontal, so the gradient is 0.
16. Harder Example
Let π π₯ = 4π₯2 β 8π₯ + 3
a) Find the gradient of π¦ = π π₯ at the point
1
2
, 0
b) Find the coordinates of the point on the graph of π¦ = π π₯ where the gradient is 8.
c) Find the gradient of π¦ = π π₯ at the points where the curve meets the line π¦ = 4π₯ β 5.
πβ²
π₯ = 8π₯ β 8
When πβ² 1
2
= 8
1
2
β 8 = β4
Remember that the βgradient functionβ allows you
to find the gradient for a particular value of π₯.
8 = 8π₯ β 8
π₯ = 2
π¦ = 4 2 2
β 8 2 + 3 = 3
Point is 2,3
This example is important!
Previously you used a value of π₯ to get the gradient πβ²
π₯ .
This time weβre doing the opposite: using a known
gradient πβ²
π₯ to get the value of π₯. We therefore
substitute πβ²
π₯ for 8.
a
b
c First find point of intersection:
4π₯2
β 8π₯ + 3 = 4π₯ β 5
Solving, we obtain: π₯ = 1 or π₯ = 2
When π₯ = 1, πβ²
1 = 0
When π₯ = 2, πβ²
2 = 8
Once you have your π₯, you need to work out π¦.
Ensure you use the correct equation!
?
?
?
17. Test Your Understanding
Let π π₯ = π₯2 β 4π₯ + 2
a) Find the gradient of π¦ = π π₯ at the point 1, β1
b) Find the coordinates of the point on the graph of π¦ = π π₯ where the gradient is 5.
c) Find the gradient of π¦ = π π₯ at the points where the curve meets the line π¦ = 2 β π₯.
πβ²
π₯ = 2π₯ β 4
When πβ² 1 = 2 1 β 4 = β2
5 = 2π₯ β 4
π₯ =
9
2
π¦ =
9
2
2
β 4
9
2
+ 2 =
17
4
Point is
9
2
,
17
4
a
b
c π₯2
β 4π₯ + 2 = 2 β π₯
Solving: π₯ = 0 or π₯ = 3
When π₯ = 0, πβ² 0 = β4
When π₯ = 3, πβ²
3 = 2
?
?
?
18. Exercise 12D
Pearson Pure Mathematics Year 1/AS
Pages 265-266
(Note that Exercise 12C was skipped in these slides)
21. Exercise 12E
Pearson Pure Mathematics Year 1/AS
Pages 267-268
Extension
[MAT 2013 1E]
The expression
π2
ππ₯2 2π₯ β 1 4 1 β π₯ 5 +
π
ππ₯
2π₯ + 1 4 3π₯2 β 2 2
is a polynomial of degree:
A) 9
B) 8
C) 7
D) less than 7
Full expansion is not needed. The highest power term in the first polynomial
is ππππ Γ βπ π = βππππ, differentiating twice to give βππ Γ π Γ π πππ.
The highest power term in the second polynomial is ππππ Γ πππ =
ππ Γ π ππ
, differentiating once to give ππ Γ π Γ π ππ
. These terms cancel
leaving a polynomial of order (at most) 6. The answer is (D).
This just means βdifferentiate twiceβ. Weβll be looking at
the βsecond derivativeβ later in this chapter.
?
22. Finding equations of tangents
(3, ? )
Find the equation of the tangent to the curve π¦ = π₯2 when π₯ = 3.
Gradient function:
π π
π π
= ππ
Gradient when π₯ = 3: π = π
π¦-value when π₯ = 3: π = π
So equation of tangent:
π β π = π π β π
?
?
?
?
We want to use π¦ β π¦1 = π π₯ β π₯1 for the tangent (as it is a straight line!).
Therefore we need:
(a) A point π₯1, π¦1
(b) The gradient π.
23. Finding equations of normals
Find the equation of the normal to the curve π¦ = π₯2 when π₯ = 3.
Equation of tangent (from earlier):
π β π = π(π β π)
Therefore equation of normal:
π β π = β
π
π
π β π
?
The normal to a
curve is the line
perpendicular to
the tangent.
(3,9)
Fro Exam Tip: A very common error is for students to
accidentally forget whether the question is asking for
the tangent or for the normal.
24. Test Your Understanding
Find the equation of the normal to the curve π¦ = π₯ + 3 π₯ when π₯ = 9.
When π₯ = 9, π¦ = 9 + 3 9 = 18
π¦ = π₯ + 3π₯
1
2
β΄
ππ¦
ππ₯
= 1 +
3
2
π₯β
1
2
ππ = 1 +
3
2
9β
1
2 =
3
2
β΄ ππ = β
2
3
Equation of normal: π¦ β 18 = β
2
3
π₯ β 9
Fro Tip: I like to use ππ and ππ to make
clear to the examiner (and myself) what
gradient Iβve found.
? y
? gradient
? Final equation
25. Exercise 12F
Pearson Pure Mathematics Year 1/AS
Pages 269-270
Extension
[STEP I 2005 Q2]
The point π has coordinates π2
, 2π and the point π
has coordinates π2
, 2π , where π and π are non-
zero and π β π. The curve πΆ is given by π¦2
= 4π₯.
The point π is the intersection of the tangent to πΆ at
π and the tangent to πΆ at π. Show that π has
coordinates ππ, π + π .
The point π is the intersection of the normal to πΆ at
π and the normal to πΆ at π. If π and π are such that
1,0 lies on the line ππ, show that π has
coordinates π2
+ π2
+ 1, π + π , and that the
quadrilateral ππππ is a rectangle.
Solutions on next slide.
[STEP I 2012 Q4]
The curve πΆ has equation π₯π¦ =
1
2
.
The tangents to πΆ at the distinct
points π π,
1
2π
and π π,
1
2π
, where
π and π are positive, intersect at π and
the normal to πΆ at these points
intersect at π. Show that π is the
point
2ππ
π + π
,
1
π + π
In the case ππ =
1
2
, find the
coordinates of π. Show (in this case)
that π and π lie on the line π¦ = π₯ and
are such that the product of their
distances from the origin is constant.
The βdifference of two cubesβ:
π3
β π3
= π β π π2
+ ππ + π2
will help for both of these.
1
2
26. Solutions to Extension Question 1
This is using something called βimplicit
differentiationβ (Year 2), but you could
easily do:
π¦2
= 4π₯ β π¦ = 2π₯
1
2
ππ¦
ππ₯
= π₯β
1
2 =
1
π₯
=
2
π¦
27. Increasing and Decreasing Functions
What do you think
it means for a
function to be an
βincreasing
functionβ?
! An increasing function
is one whose gradient is
always at least 0.
πβ²
π₯ β₯ 0 for all π₯.
It would be βstrictly increasingβ
if π π₯ > 0 for all π₯, i.e. is not
allowed to go horizontal.
A function can also be increasing
and decreasing in certain intervals.
2,3
4, β1
Increasing
for π₯ β€ 2
Decreasing for
2 β€ π₯ β€ 4
Increasing
for π₯ β₯ 4
We could also write βπ π₯ is
decreasing in the interval [2,4]β
[π, π] represents all the real numbers
between π and π inclusive, i.e:
π, π = π₯ βΆ π β€ π₯ β€ π
? ? ?
28. Examples
Show that the function
π π₯ = π₯3 + 6π₯2 + 21π₯ + 2 is
increasing for all real values of π₯.
πβ² π₯ = 3π₯2 + 12π₯ + 21
πβ² π₯ = 3 π₯2 + 4π₯ + 7
= 3 π₯ + 2 2 + 9
π₯ + 2 2
β₯ 0 for all real π₯,
β΄ 3 π₯ + 2 2
+ 9 β₯ 0 for all real π₯
β΄ π(π₯) is an increasing function for
all π₯.
Fro Tip: To show a quadratic is
always positive, complete the
square, then indicate the
squared term is always at
least 0.
?
Find the interval on which the
function π π₯ = π₯3 + 3π₯2 β 9π₯
is decreasing.
π π₯ = π₯3
+ 3π₯2
β 9π₯
πβ² π₯ = 3π₯2 + 6π₯ β 9
πβ²
π₯ β€ 0
3π₯2
+ 6π₯ β 9 β€ 0
π₯2 + 2π₯ β 3 β€ 0
π₯ + 3 π₯ β 1 β€ 0
β3 β€ π₯ β€ 1
So π(π₯) is decreasing in the
interval [β3,1]
?
29. Test Your Understanding
Show that the function
π π₯ = π₯3 + 16π₯ β 2 is
increasing for all real values of π₯.
Find the interval on which the
function π π₯ = π₯3 + 6π₯2 β 135π₯
is decreasing.
πβ²
π₯ = 3π₯2
+ 16
π₯2 β₯ 0 for all real π₯
β΄ 3π₯2
+ 16 β₯ 0 for all real π₯.
Therefore π(π₯) is an increasing
function for all real π₯.
πβ²
π₯ = 3π₯2
+ 12π₯ β 135
πβ² π₯ β€ 0
β΄ 3π₯2 + 12π₯ β 135 β€ 0
π₯2
+ 4π₯ β 45 β€ 0
π₯ + 9 π₯ β 5 β€ 0
β9 β€ π₯ β€ 5
So π(π₯) is decreasing in the
interval [β9,5]
? ?
31. Second Order Derivatives
When you differentiate once, the expression you get is known as the first derivative.
Unsurprisingly, when we differentiate a second time, the resulting expression is
known as the second derivative. And so onβ¦
π¦ = π₯4
ππ¦
ππ₯
= 4π₯3
π¦β² = 4π₯3
π¦ = 4π₯3
π2π¦
ππ₯2 = 12π₯2
π¦β²β² = 12π₯2
π¦ = 12π₯2
π π₯ = π₯4 πβ² π₯ = 4π₯3 πβ²β² π₯ = 12π₯2
Lagrangeβs
Original Function First Derivative Second Derivative
You can similarly have the third derivative (
π3π¦
ππ₯3), although this is no longer in the A
Level syllabus. Weβll see why might use the second derivative soonβ¦
32. Just for your interestβ¦
How does the notation
π2π¦
ππ₯2 work?
Why are the squareds where they are?
Suppose that π¦ = π₯3
+ 1.
Then when we write
ππ¦
ππ₯
, weβre
effectively doing
π π₯3+1
ππ₯
(by
substitution), although this would
typically be written:
π
ππ₯
π₯3
+ 1
The
π
ππ₯
(β¦ ) notation is quite handy,
because it behaves as a function and
allows us to write the original
expression and the derivative within
a single equation:
π
ππ₯
π₯3
+ 1 = 3π₯2
Therefore, if we wanted to differentiate π¦ twice, weβd do:
π
ππ₯
π
ππ₯
π¦
=
π2
ππ₯2
π¦
=
π2
π¦
ππ₯2
ME-WOW!
34. Test Your Understanding
If π¦ = 5π₯3 β
π₯
3 π₯
, find
π2π¦
ππ₯2.
π¦ = 5π₯3 β
1
3
π₯
1
2
ππ¦
ππ₯
= 15π₯2
β
1
6
π₯β
1
2
π2
π¦
ππ₯2 = 30π₯ +
1
12
π₯β
3
2
?
(Note: For time reasons, Iβm skipping Exercise 12H on Page 272)
Also note an error on this page: βWhen you differentiate with respect to π₯, you treat any other letters as constants.β
This is emphatically not true β it depends on whether the letter is a variable or a constant.
The statement however is true of βpartial differentiationβ (which is not in the A Level syllabus). In fact in Year 2, you will learn that π₯π¦,
when differentiated with respect to π₯, gives π₯
ππ¦
ππ₯
+ π¦, not to just π¦. What the textbook means is βWhen you differentiate with respect
to π₯, you treat any letters, defined to be constants, as numbers.β So ππ₯ would differentiate to π, just as 3π₯ differentiates to 3, but ONLY
if you were told that π is a constant! If π was in fact a variable, we need to use something called the product rule (Year 2 content).
35. Stationary/Turning Points
A stationary point is where the gradient is 0, i.e. πβ²
π₯ = 0.
πβ²
π₯ = 0
πβ²
π₯ = 0
Local maximum
Local minimum
Fro Note: Itβs called a βlocalβ maximum
because itβs the functionβs largest
output within the vicinity. Functions
may also have a βglobalβ maximum, i.e.
the maximum output across the entire
function. This particular function
doesnβt have a global maximum
because the output keeps increasing up
to infinity. It similarly has no global
minimum, as with all cubics.
Find the coordinates of the turning points of π¦ = π₯3 + 6π₯2 β 135π₯
ππ¦
ππ₯
= 3π₯2
+ 12π₯ β 135 = 0
π₯2
+ 4π₯ β 45 = 0
π₯ + 9 π₯ β 5 = 0
π₯ = β9 ππ π₯ = 5
When π₯ = 5, π¦ = 53
+ 6 52
β 135 5 = β400 β 5, β400
When π₯ = β9, π¦ = β9 3
+ 6 β9 2
β 135 β9 = 972 β (β9,972)
?
36. More Examples
Find the least value of
π π₯ = π₯2 β 4π₯ + 9
Method 1: Differentiation
πβ²
π₯ = 2π₯ β 4 = 0
π₯ = 2
π 2 = 22 β 4 2 + 9 = 5
So 5 is the minimum value.
Method 2: Completing the square
π π₯ = π₯ β 2 2 + 5
Therefore the minimum value of
π(π₯) is 5, and this occurs when
π₯ = 2.
?
Method 1:
Differentiation
?
Method 2:
Completing the Square
Fro Note: Method 2 is only applicable for
quadratic functions. For others,
differentiation must be used.
Find the turning point of
π¦ = π₯ β π₯
π¦ = π₯
1
2 β π₯
ππ¦
ππ₯
=
1
2
π₯β
1
2 β 1 = 0
1
2
π₯β
1
2 = 1
π₯β
1
2 = 2 β
1
π₯
= 2
π₯ =
1
2
β π₯ =
1
4
When π₯ =
1
4
,
π¦ =
1
4
β
1
4
=
1
4
So turning point is
1
4
,
1
4
?
37. Points of Inflection
Thereβs a third type of stationary point (that weβve encountered previously):
πβ²
π₯ = 0
A point of inflection is where
the curve changes from convex
concave (or vice versa).
convex
concave
(the same terms
used in optics!)
i.e. the line curves in one
direction before the point of
inflection, then curves in the
other direction after.
Technically we could label
these either way round
depending on where we view
the curve from. Whatβs
important is that the
concavity changes.
Fro Side Note: Not all points of inflection are stationary points, as can be seen
in the example on the right.
A point of inflection which is a stationary point is known as a saddle point.
38. How do we tell what type of stationary point?
Local Minimum
Gradient
just before
Gradient at
minimum
Gradient
just after
-ve 0 +ve
Point of Inflection
Gradient
just before
Gradient at
p.o.i
Gradient
just after
+ve 0 +ve
Local Maximum
Gradient
just before
Gradient at
maximum
Gradient
just after
+ve 0 +ve
? ?
? ?
? ?
?
?
?
Method 1: Look at gradient just before
and just after point.
39. How do we tell what type of stationary point?
Method 1: Look at gradient just before
and just after point.
Find the stationary point on the curve with equation
π¦ = π₯4 β 32π₯, and determine whether it is a local maximum, a local
minimum or a point of inflection.
Strategy: Find the gradient for values just before and after π₯ = 2. Letβs try π₯ = 1.9 and π₯ = 2.1.
ππ¦
ππ₯
= 4π₯3
β 32 = 0
π₯ = 2 β΄ π¦ = β48
Stationary point is 2, β48
π = π. π π = π π = π. π
Gradient β4.56 0 5.04
Shape
Looking at the shape, we can see that 2, β48 is a minimum.
? Turning Point
? Determine point type
?
40. Method 2: Using the second derivative
The method of substituting values of π₯ just before and after is a bit cumbersome.
It also has the potential for problems: what if two different types of stationary points
are really close together?
Recall the gradient gives a
measure of the rate of change
of π¦, i.e. how much the π¦ value
changes as π₯ changes.
Thus by differentiating the
gradient function, the second
derivative tells us the rate at
what the gradient is changing.
Thus if the second derivative is
positive, the gradient is
increasing.
If the second derivative is
negative, the gradient is
decreasing.
+ve
gradient
0
gradient
-ve
gradient
At a maximum point, we can see
that as π₯ increases, the gradient
is decreasing from a positive
value to a negative value.
β΄
π2
π¦
ππ₯2
< 0
41. Method 2: Using the second derivative
! At a stationary point π₯ = π:
β’ If πβ²β² π > 0 the point is a local minimum.
β’ If πβ²β² π < 0 the point is a local maximum.
β’ If πβ²β² π = 0 it could be any type of point, so resort to Method 1.
I will eventually do a βJust for your
Interestβ¦β thingy on why we canβt
classify the point when πβ²β²
π₯ = 0,
and how we could use the third
derivative!
The stationary point of π¦ = π₯4 β 32π₯ is
2, β48 . Use the second derivative to classify
this stationary point.
ππ¦
ππ₯
= 4π₯3
β 32
π2
π¦
ππ₯2 = 12π₯2
When π₯ = 2,
π2π¦
ππ₯2 = 12 2 2 > 0
Therefore the stationary point is a minimum point.
Find
π2π¦
ππ₯2 for the π₯ at the
stationary point.
?
43. Sketching Graphs
All the way back in Chapter 4, we used features such as intercepts with the
axes, and behaviour when π₯ β β and π₯ β ββ in order to sketch graphs.
Now we can also find stationary/turning points!
[Textbook] By first finding the stationary points, sketch the graph of π¦ =
1
π₯
+ 27π₯3
π¦ = π₯β1 + 27π₯3
ππ¦
ππ₯
= βπ₯β2
+ 81π₯2
= 0
β
1
π₯2
+ 81π₯2
= 0 β 81π₯2
=
1
π₯2
81π₯4
= 1 β π₯ =
1
3
ππ β
1
3
π
π
, π , β
π
π
, βπ
As π₯ β β, π β β
As π₯ β ββ, π β ββ
π₯ not defined at 0 (due to
1
π₯
term)
? Vertical Asymptotes
? As π₯ β β, π₯ β ββ
? Turning Points
π₯
π¦
1
3
4
β4
β
1
3
? Graph
44. Exercise 12I
Pearson Pure Mathematics Year 1/AS
Page 276
Extension
[MAT 2014 1C] The cubic
π¦ = ππ₯3 β π + 1 π₯2 + 2 β π π₯ β π
has a turning point, that is a minimum,
when π₯ = 1 precisely for
A) π > 0
B) 0 < π < 1
C) π >
1
2
D) π < 3
E) all values of π
[MAT 2004 1B] The smallest value of
the function:
π π₯ = 2π₯3
β 9π₯2
+ 12π₯ + 3
In the range 0 β€ π₯ β€ 2 is what?
[MAT 2001 1E] The maximum gradient of the curve
π¦ = π₯4
β 4π₯3
+ 4π₯2
+ 2 in the range 0 β€ π₯ β€ 2
1
5
occurs when:
A) π₯ = 0
B) π₯ = 1 β
1
3
C) π₯ = 1 +
1
3
D) π₯ = 2
1
5
[STEP I 2007 Q8] A curve is given by:
π¦ = ππ₯3 β 6ππ₯2 + 12π + 12 π₯ β (8π + 16)
where π is a real number. Show that this curve
touches the curve with equation π¦ = π₯3
at
2,8 . Determine the coordinates of any other
point of intersection of the two curves.
(i) Sketch on the same axes these two curves
when π = 2.
(ii) β¦ when π = 1 (iii) when π = β2
Hint: When two curves
touch, their π¦ values
must match, but what
else must also match?
1
2
3
4
45. Solutions to Extension Questions
[MAT 2014 1C] The cubic
π¦ = ππ₯3 β π + 1 π₯2 + 2 β π π₯ β π
has a turning point, that is a minimum,
when π₯ = 1 precisely for
A) π > 0
B) 0 < π < 1
C) π >
1
2
D) π < 3
E) all values of π
π π
π π
= ππππ
β π π + π π + (π β π)
When π = π,
π π
π π
= ππ β ππ β π + π β π β‘ π
Therefore there is a turning point for all values of
π. However, this must be a minimum.
π π
π
π ππ
= πππ β π(π + π)
When π = π,
π ππ
π ππ = ππ β π
If minimum: ππ β π > π β π >
π
π
[MAT 2004 1B] The smallest value of
the function:
π π₯ = 2π₯3 β 9π₯2 + 12π₯ + 3
In the range 0 β€ π₯ β€ 2 is what?
πβ² π = πππ β πππ + ππ = π
ππ
β ππ + π = π
π β π π β π = π
π = π β π π = π
π = π β π π = π
At start of range: π π = π
(0,3)
(1,8)
(2,7)
Therefore answer is 3.
1 2
46. Solutions to Extension Questions
[MAT 2001 1E] The maximum gradient of the
curve π¦ = π₯4
β 4π₯3
+ 4π₯2
+ 2 in the range
0 β€ π₯ β€ 2
1
5
occurs when:
A) π₯ = 0
B) π₯ = 1 β
1
3
C) π₯ = 1 +
1
3
D) π₯ = 2
1
5
Gradient:
π π
π π
= πππ
β ππππ
+ ππ
So find max value of πππ
β ππππ
+ ππ
π π
π
π ππ
= ππππ
β πππ + π = π
πππ
β ππ + π = π
π = π Β±
π
π
Due to the shape of a cubic, we have the a local
maximum gradient at π = π β
π
π
and a local minimum at π = π +
π
π
In range π β€ π β€ π
π
π
, answer must either be π = π +
π
π
or π = π
π
π
. Substituting these into π yields a higher
value for the latter, so answer is (D).
3
47. [STEP I 2007 Q8] A curve is
given by:
π¦ = ππ₯3
β 6ππ₯2
+ 12π + 12 π₯ β (8π + 16)
where π is a real number.
Show that this curve touches
the curve with equation π¦ =
π₯3
at 2,8 . Determine the
coordinates of any other point
of intersection of the two
curves.
(i) Sketch on the same axes
these two curves when π = 2.
(ii) β¦ when π = 1
(iii) β¦ when π = β2
Solutions to Extension Questions
4
48. Sketching Gradient Functions
The new A Level
specification specifically
mentions being able to
sketch π¦ = πβ²(π₯).
If you know the function
πβ²(π₯) explicitly (e.g.
because you differentiated
π¦ = π(π₯)), you can use
your knowledge of
sketching straight
line/quadratic/cubic
graphs.
But in other cases you
wonβt be given the
function explicitly, but just
the sketch.
π₯
π¦
Click to Sketch
π¦ = πβ²(π₯)
Click to Sketch
π¦ = πβ²(π₯)
Click to Sketch
π¦ = πβ²(π₯)
The gradient at the
turning point is 0.
The gradient of
π¦ = π π₯ is
negative, but
increasing.
The gradient
of π¦ = π π₯ is
positive, and
increasing.
49. A Harder One
π₯
π¦
> > > > > > >
π¦ = π π₯
π¦ = πβ²(π₯)
Gradient is
negative, but
increasing.
Fro Tip: Mentally describe
the gradient of each section
of the curve, starting your
mental sentence with βThe
gradient isβ¦β and using
terms like βpositiveβ, βbut
increasingβ, βbut not
changingβ, etc.
Gradient
is 0.
Gradient is
positive: initially
increases but
then decreases.
Gradient is 0,
but positive
before and after.
Gradient is
positive: initially
increases but
then decreases.
Gradient is 0.
Gradient is negative. Initially
decreases but then tends towards 0.
Fro Pro Tip: The gradient is momentarily not changing so
the gradient of the gradient is 0. We get a turning point in
π¦ = πβ²
π₯ whenever the curve has a point of inflection.
Again, on π¦ = π(π₯) this is a point of inflection so the
gradient function has a turning point. It is also a stationary
point, so πβ²
π₯ = 0. i.e. The πβ²(π₯) curve touches the π₯-axis.
50. Test Your Understanding
π₯
π¦
Solution
>
π¦ = π π₯
π¦ = πβ²(π₯)
Gradient is
negative, but
increasing.
Gradient is 0.
Gradient is positive;
initially increases
but then decreases.
Gradient is positive, but
tending towards 0.
51. Exercise 12J
Pearson Pure Mathematics Year 1/AS
Page 278
Extension
[MAT 2015 1B]
π π₯ = π₯ + π π
where π is a real number and π is a
positive whole number, and π β₯ 2. If
π¦ = π(π₯) and π¦ = πβ²(π₯) are plotted on
the same axes, the number of
intersections between π(π₯) and πβ² π₯
will:
A) always be odd
B) always be even
C) depend on π but not π
D) depend on π but not π
E) depend on both π and π.
1
(Official solution)
If π(π) is an even function
then πβ² π will be an odd
function, and vice versa. π(π)
and πβ²
π will cross the π-axis
at βπ, π and will have one
further intersection when π
and π are greater than 0.
The answer is (b).
Recall that:
Odd function: π βπ₯ = βπ π₯
Even function: π βπ₯ = π(π₯)
?
52. These are examples of optimisation
problems: weβre trying to
maximise/minimise some quantity
by choosing an appropriate value of
a variable that we can control.
We have a sheet of A4 paper,
which we want to fold into a
cuboid. What height should we
choose for the cuboid in order to
maximise the volume?
π₯
π¦
We have 50m of
fencing, and want to
make a bear pen of the
following shape, such
that the area is
maximised. What
should we choose π₯
and π¦ to be?
Optimisation Problems/Modelling
β
53. βRate of changeβ
Up to now weβve had π¦ in terms of π₯, where
ππ¦
ππ₯
means βthe rate at which π¦
changes with respect to π₯β.
But we can use similar gradient function notation for other physical quantities.
Real Life
Maths!
A sewage container fills at a
rate of 20 cm3 per second.
How could we use
appropriate notation to
represent this?
ππ
ππ‘
= 20 ππ3/π
βThe rate at which the volume π
changes with respect to time π‘.β
54. Example Optimisation Problem
Optimisation problems in an exam usually follow the following pattern:
β’ There are 2 variables involved (you may have to introduce one yourself), typically lengths.
β’ There are expressions for two different physical quantities:
β’ One is a constraint, e.g. βthe surface area is 20cm2β.
β’ The other we wish to maximise/minimise, e.g. βwe wish to maximise the volumeβ.
β’ We use the constraint to eliminate one of the variables in the latter equation, so that it is then just in
terms of one variable, and we can then use differentiation to find the turning point.
π
π
π
[Textbook] A large tank in the shape of a cuboid is to be
made from 54m2 of sheet metal. The tank has a horizontal
base and no top. The height of the tank is π₯ metres. Two of
the opposite vertical faces are squares.
a) Show that the volume, V m3, of the tank is given by
π = 18π₯ β
2
3
π₯3
.
We need to introduce a second
variable ourselves so that we
can find expressions for the
surface area and volume.
b) Given that π₯ can vary, use differentiation to find
the maximum or minimum value of π.
2π₯2
+ 3π₯π¦ = 54
π = π₯2
π¦
But we want π just in terms of π₯:
π¦ =
54 β 2π₯2
3π₯
β π = π₯2
54 β 2π₯2
3π₯
π =
54π₯2
β 2π₯4
3π₯
= 18π₯ β
2
3
π₯3
These are the two equations
mentioned in the guidance: one
for surface area and one volume.
ππ
ππ‘
= 18 β 2π₯2
= 0 β΄ π₯ = 3
π = 18 3 β
2
3
3 3
= 36
Once we have the
βoptimalβ value of π₯, we
sub it back into π to get
the best possible
volume.
?
?
56. Exercise 12K
Pearson Pure Mathematics Year 1/AS
Page 281
Extension
[STEP I 2006 Q2] A small goat is tethered by a rope to a point at ground
level on a side of a square barn which stands in a large horizontal field
of grass. The sides of the barn are of length 2π and the rope is of length
4π. Let π΄ be the area of the grass that the goat can graze.
Prove that π΄ β€ 14ππ2 and determine the minimum value of π΄.
1
?