1. “Teach A Level Maths” Vol. 1: AS Core Modules
12: Tangents and Gradients
© Christine Crisp

2. Module C1 Module C2 AQA MEI/OCR Edexcel OCR
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3. We need to be able to find these points using algebra
Points with a Given Gradient
e.g. Find the coordinates of the points on the curve
where the gradient equals 4
Gradient of curve
= gradient of tangent
= 4
We need to be able to find these points using algebra

4. Points with a Given Gradient
e.g. Find the coordinates of the points on the curve
where the gradient is 4
The gradient of the curve is given by
Solution:
Quadratic equation with no linear x -term
Gradient is 4

5. Points with a Given Gradient
The points on with gradient 4

6. SUMMARY
To find the point(s) on a curve with a given gradient:
find the gradient function
let equal the given gradient
solve the resulting equation

7. Exercises
Find the coordinates of the points on the curves with the gradients given 1.
where the gradient is -2
Ans: (-3, -6) 2.
where the gradient is 3
( Watch out for the common factor in the quadratic equation )
Ans: (-2, 2) and (4, -88)

8. Increasing and Decreasing Functions
An increasing function is one whose gradient is always greater than or equal to zero.
for all values of x
A decreasing function has a gradient that is always negative or zero.
for all values of x

9. e.g.1 Show that is an increasing function
Solution:
is the sum of
a positive number ( 3 )  a perfect square ( which is positive or zero for all values of x, and
a positive number ( 4 )
for all values of x
so, is an increasing function

10. e.g.2 Show that is an increasing function.
Solution:
To show that is never negative ( in spite of the negative term ), we need to complete the square.
for all values of x
Since a square is always greater than or equal to zero,
is an increasing function.

11. The graphs of the increasing functions and are

12. Exercises
1. Show that is a decreasing function and sketch its graph.
2. Show that is an increasing function and sketch its graph.
Solutions are on the next 2 slides.

13. Solutions
1. Show that is a decreasing function and sketch its graph.
Solution: This is the product of a square which is always
and a negative number,
so for all x. Hence is a decreasing function.

14. Solutions
2. Show that is an increasing function and sketch its graph.
Solution:
Completing the square:
which is the sum of a square which is
and a positive number. Hence y is an increasing function.

15. (-1, 3) on line: The equation of a tangent
e.g. 1 Find the equation of the tangent at the point
(-1, 3) on the curve with equation
Solution:
The gradient of a curve at a point and the gradient of the tangent at that point are equal
(-1, 3) x
Gradient = -5
At x = -1
(-1, 3) on line:
So, the equation of the tangent is

16. ( We read this as “ f dashed x ” )
An Alternative Notation
The notation for a function of x can be used instead of y.
When is used, instead of using for the gradient function, we write
( We read this as “ f dashed x ” )
This notation is helpful if we need to substitute for x.
e.g.

17. e.g. 2 Find the equation of the tangent where x = 2 on the curve with equation where
Solution: To use we need to know y at the point as well as x and m
From (1),
(2, 2) on the line
So, the equation of the tangent is

18. SUMMARY To find the equation of the tangent at a point on the curve :
if the y-value at the point is not given, substitute the x -value into the equation of the curve to find y
find the gradient function
substitute the x-value into
to find the gradient of the tangent, m
substitute for y, m and x into to find c

19. Exercises
Find the equation of the tangent to the curve 1.
at the point (2, -1)
Ans: 2.
Find the equation of the tangent to the curve
at the point x = -1, where
Ans:

21. The following slides contain repeats of information on earlier slides, shown without colour, so that they can be printed and photocopied.
For most purposes the slides can be printed as “Handouts” with up to 6 slides per sheet.

22. SUMMARY
To find the point(s) on a curve with a given gradient:
let equal the given gradient
solve the resulting equation
find the gradient function

23. Gradient is 4
e.g. Find the coordinates of the points on the curve
where the gradient is 4
The gradient of the curve is given by
Solution:
Quadratic equation with no linear x -term

24. Increasing and Decreasing Functions
An increasing function is one whose gradient is always greater than or equal to zero.
for all values of x
A decreasing function has a gradient that is always negative or zero.

25. Solution:
e.g. Show that is an increasing function.
To show that is never negative ( in spite of the negative term ), we need to complete the square.
is an increasing function.
for all values of x
Since a square is always greater than or equal to zero,

26. SUMMARY To find the equation of the tangent at a point on the curve :
if the y-value at the point is not given, substitute the x -value into the equation of the curve to find y
To find the equation of the tangent at a point on the curve :
find the gradient function
substitute the x-value into
to find the gradient of the tangent, m
substitute for y, m and x into to find c

27. (-1, 3) on line: The equation of a tangentx
Solution:
At x = -1
So, the equation of the tangent is
Gradient = -5
(-1, 3) on line:
e.g. 1 Find the equation of the tangent at the point
(-1, 3) on the curve with equation
The equation of a tangent

28. Solution: To use we need to know y at the point as well as x and m
So, the equation of the tangent is
From (1),
(2, 2) on the line
e.g. 2 Find the equation of the tangent where x = 2 on the curve with equation where