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Stretches of Graphs (DP IB Maths: AI HL)
Revision Note
Stretches of Graphs
What are stretches of graphs?
- When you alter a function in certain ways, the effects on the graph of the function can be described by geometrical transformations
- For a stretch:
- the graph is stretched about one of the coordinate axes by a scale factor
- Its size changes
- the orientation of the graph remains unchanged
- the graph is stretched about one of the coordinate axes by a scale factor
- A particular stretch is specified by a coordinate axis and a scale factor:
- The distance between a point on the graph and the specified coordinate axis is multiplied by the constant scale factor
- The graph is stretched in the direction which is parallel to the other coordinate axis
- For scale factors bigger than 1
- the points on the graph get further away from the specified coordinate axis
- For scale factors between 0 and 1
- the points on the graph get closer to the specified coordinate axis
- This is also sometimes called a compression but in your exam you must use the term stretch with the appropriate scale factor
What effects do horizontal stretches have on the graphs and functions?
- A horizontal stretch of the graph by a scale factor q centred about the y-axis is represented by
- The x-coordinates change
- They are divided by q
- The y-coordinates stay the same
- The coordinates become
- Horizontal asymptotes stay the same
- Vertical asymptotes change
- becomes
What effects do vertical stretches have on the graphs and functions?
- A vertical stretch of the graph by a scale factor p centred about the x-axis is represented by
- This is often rearranged to
- The x-coordinates stay the same
- The y-coordinates change
- They are multiplied by p
- The coordinates become
- Horizontal asymptotes change
- becomes
- Vertical asymptotes stay the same
Examiner Tip
- To get full marks in an exam make sure you use correct mathematical terminology
- For example: Stretch vertically by scale factor ½
- Do not use the word "compress" in your exam
Worked example
The diagram below shows the graph of .
a)
Sketch the graph of .
b)
Sketch the graph of .
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