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

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The graphs are all symmetrical about the y axis.

Any function which is symmetrical about the y 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

Even Function

An even function is such that f ( x ) = f ( x ) for all values of x .

Key Point 4 is saying that the function value at a negative value of x is the same as the function value at the corresponding positive value of x .

Example 8

Show algebraically that f ( x ) = x 4 + 5 is an even function.

Solution

We must show that f ( x ) = f ( x ) .

f ( x ) = ( x ) 4 + 5 = x 4 + 5

Hence f ( x ) = f ( x ) and so the function is even. Check for yourself that f ( 3 ) = f ( 3 ) .

Task!

Extend the graph in the solution box in order to produce a graph of an even function.

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Task!

The following diagrams shows graphs of several functions. They share a common property. Study the graphs and comment on any symmetry.

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There is rotational symmetry about the origin. That is, each curve, when rotated through 18 0 ∘ , 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 18 0 ∘ about the origin is said to be an odd function. Odd functions have the following property:

Key Point 5

Odd Function

An odd function is such that f ( x ) = f ( x ) for all values of x .

Key Point 5 is saying that the function value at a negative value of x is minus the function value at the corresponding positive value of x .

Example 9

Show that the function f ( x ) = x 3 + 4 x is odd.

Solution

We must show that f ( x ) = f ( x ) .

f ( x ) = ( x ) 3 + 4 ( x ) = x 3 4 x = ( x 3 + 4 x ) = f ( x )

and so this function is odd. Check for yourself that f ( 2 ) = f ( 2 ) .

Task!

Extend the graph in the solution box in order to produce a graph of an odd function.

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Note that some functions are neither odd nor even ; for example f ( x ) = x 3 + x 2 is neither even nor odd.

The reader should confirm (with simple examples) that, ‘odd’ and ‘even’ functions have the following properties:

odd + odd = odd         even + even = even         odd + even = neither odd × odd = even even × even = even odd × even = odd
Exercises
  1. Classify the following functions as odd, even or neither. If necessary sketch a graph to help you decide.
    1. f ( x ) = 6 ,
    2. f ( x ) = x 2 ,
    3. f ( x ) = 2 x + 1 ,
    4. f ( x ) = x ,
    5. f ( x ) = 2 x
  2. 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, f ( x ) = cosh x , discussed in HELM booklet  6.

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    A catenary

    1. Comment upon any symmetry.
    2. Is this function one-to-one or many-to-one?
    3. Is this a continuous or discontinuous function?
    4. State lim x 0 cosh x .
    1. even,
    2. even,
    3. neither,
    4. odd,
    5. odd
    1. function is even, symmetric about the y -axis,   
    2. many-to-one,  
    3. continuous,  
    4. 1