3 Odd and even functions
Example 7
Figure 23 shows graphs of several functions. They share a common property. Study the graphs and comment on any symmetry.
Figure 23
The graphs are all symmetrical about the axis.
Any function which is symmetrical about the axis, i.e. where the graph of the right-hand part is the mirror image of that on the left, is said to be an even function . Even functions have the following property:
Key Point 4 is saying that the function value at a negative value of is the same as the function value at the corresponding positive value of .
Example 8
Show algebraically that is an even function.
Solution
We must show that .
Hence and so the function is even. Check for yourself that .
Task!
Extend the graph in the solution box in order to produce a graph of an even function.
Task!
The following diagrams shows graphs of several functions. They share a common property. Study the graphs and comment on any symmetry.
There is rotational symmetry about the origin. That is, each curve, when rotated through , transforms into itself.
Any function which possesses such symmetry that is the graph of the right can be obtained by rotating the curve on the left through about the origin is said to be an odd function. Odd functions have the following property:
Key Point 5 is saying that the function value at a negative value of is minus the function value at the corresponding positive value of .
Example 9
Show that the function is odd.
Solution
We must show that .
and so this function is odd. Check for yourself that .
Task!
Extend the graph in the solution box in order to produce a graph of an odd function.
Note that some functions are neither odd nor even ; for example is neither even nor odd.
The reader should confirm (with simple examples) that, ‘odd’ and ‘even’ functions have the following properties:
Exercises
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Classify the following functions as odd, even or neither. If necessary sketch a graph to help you decide.
- ,
- ,
- ,
- ,
-
The diagram below represents a heavy cable hanging under gravity from two points at the same height. Such a curve (shown as a dashed line), known as a catenary , is described by a mathematical function known as a hyperbolic cosine, , discussed in HELM booklet 6.
A catenary
- Comment upon any symmetry.
- Is this function one-to-one or many-to-one?
- Is this a continuous or discontinuous function?
- State .
-
- even,
- even,
- neither,
- odd,
- odd
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- function is even, symmetric about the -axis,
- many-to-one,
- continuous,
- 1