Stretches of Graphs (Edexcel IGCSE Maths)

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Daniel I

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Daniel I

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Stretches of Graphs

How do we stretch graphs of functions?

In relation to y = f(x);

  • y = f(ax) is a horizontal stretch/ stretch in the x-axis by a scale factor of 1/a
    • May also be referred to as a stretch in (or parallel to) the x-direction
    • This looks like a “squash” but that is not a technical term – do not use it!
    • To perform a stretch in the x-axis given by f(ax);
      1. pick key points on the original graph like end points, turning points and axis intercepts
      2. multiply the x-coordinate of these points by 1/(or divide by a). Do not change the y-coordinate!
      3. plot the new points and join with a curved or straight line as appropriate

y = af(x) is a vertical stretch/ stretch in the y-axis by a scale factor of a

  • May also be referred to as a stretch in (or parallel to) the y-direction
  • To perform a stretch in the y-axis given by af(x);
    1. pick key points on the original graph like end points, turning points and axis intercepts
    2. multiply the y-coordinate of these points by a. Do not change the x-coordinate
    3. plot the new points and join with a curved or straight line as appropriate

edexcel-igcse-3-graphs-transformations-stretches

  • As with translations and reflections, trig graphs are a common subject for transformation questions

GT Notes fig5, downloadable IGCSE & GCSE Maths revision notes

How are stretches and reflections of graphs related?

  • We have previously seen that f(-x) and -f(x) are reflections in the x-axis and the y-axis respectively
  • Both of the above are also special cases of stretches, f(ax) and af(x) where a = -1 

How do stretches of graphs work?

  • There is a logic to these as well but memory and recognition is easier!
  • With y = af(x) the “a” happens after the function
    • So the ‘output’ of the function changes

      ie. y-coordinates change

  • With y = f(ax) the “a” happens before the function
    • So the ‘input’ of the function changes

      ie. x-coordinates change

    • This stretch may seem it has the wrong scale factor (1/a)
      • However the ‘input’ is being multiplied by a – so x will need to be divided by a in order for the function to receive the same input

How do I describe stretches of graphs?

  • Some questions give a transformed function in the form y = f(ax) or y = af(x) and ask you to describe the transformation
  • To describe a stretch fully, you must include;
    • the transformation: "stretch"
    • the direction: "in the x-axis" / "in the y-axis"
    • the stretch factor: "by 1/a" / "by a"

GT Notes fig8, downloadable IGCSE & GCSE Maths revision notes

Examiner Tip

REMEMBER that;

  • y = f(ax);  "a" next to x, stretches in x-axis by 1/a
  • y = af(x);  "a" not next to x, stretches in y-axis

Worked example

(a)
The graph of y equals straight f open parentheses x close parentheses is shown.
edexcel-igcse-3-graphs-transformations-stretches1

Sketch the graph of y equals 2 straight f open parentheses x close parentheses on the grid.

y = 2f(x) represents a stretch in the y-axis by factor 2

First pick the key points- axis intercepts at (0, 0), a turning point at (1, -1) and integer points at (-1, 3) and (3, 3)
For each point, keep its x-coordinate the same but multiply its y-coordinate by 2
Mark the new points on the grid
edexcel-igcse-3-graphs-transformations-stretches2

Notice that points on the x-axis do not change during a stretch in the y-axis (they are invariant)
Join the new points with a smooth line. You must make sure that your stretched curve passes through the points marked on the answer diagram below

edexcel-igcse-3-graphs-transformations-stretches3
(b)
The diagram shows part of the curve with equation y equals f open parentheses x close parentheses.
The coordinates of the maximum point of the curve are open parentheses 6 comma space 15 close parentheses.
edexcel-igcse-3-graphs-transformations-stretches4
State the coordinates of the maximum point of the curve with equation y equals f open parentheses 3 x close parentheses.

y=f(3x) represents a stretch in the x-axis by a factor of 1/3
The y-coordinate of (6, 15) does not change but the x-coordinate is multiplied by 1/3 (divided by 3)

6 × (1/3) = 2
(2, 15)

Worked example

(a)
Describe the transformation that maps the graph of y equals f open parentheses x close parentheses to the graph of y equals f open parentheses 2 x close parentheses.

The number is inside the bracket (next to x)
Stretch in x-axis by 1/2

(b)
Describe the transformation that maps the graph of y equals f open parentheses x close parentheses to the graph of y equals 3 f open parentheses x close parentheses.

The number is outside the bracket 
Stretch in y-axis by 3

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Daniel I

Author: Daniel I

Expertise: Maths

Daniel has taught maths for over 10 years in a variety of settings, covering GCSE, IGCSE, A-level and IB. The more he taught maths, the more he appreciated its beauty. He loves breaking tricky topics down into a way they can be easily understood by students, and creating resources that help to do this.