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);
- pick key points on the original graph like end points, turning points and axis intercepts
- multiply the x-coordinate of these points by 1/a (or divide by a). Do not change the y-coordinate!
- 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);
- pick key points on the original graph like end points, turning points and axis intercepts
- multiply the y-coordinate of these points by a. Do not change the x-coordinate
- plot the new points and join with a curved or straight line as appropriate
- As with translations and reflections, trig graphs are a common subject for transformation questions
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
- So the ‘output’ of the function changes
- 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
- So the ‘input’ of the function changes
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"
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 is shown.
Sketch the graph of 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
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
(b)
The diagram shows part of the curve with equation .
The coordinates of the maximum point of the curve are .
The coordinates of the maximum point of the curve are .
State the coordinates of the maximum point of the curve with equation .
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 to the graph of .
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 to the graph of .
The number is outside the bracket
Stretch in y-axis by 3