EVEN AND ODD FUNCTIONS

Even and Odd Functions :

Let f(x) be a function. 

To find whether f(x) is even or odd, we have to replace "x" by "-x" in f(x). We have to conclude f(x) as even or odd function from the result of f(-x) as shown below.   

1. If f(-x)  =  f(x), then f(x) is even function

2. If f(-x)  =  - f(x), then f(x) is odd function

If f(-x) is neither equal to f(x) nor -f(x), we have to conclude f(x) is neither even nor odd. 

Even and Odd Functions - Application

Even though we have many applications of even and odd functions, let us consider the important application of even and odd functions in integral calculus.  

If f(x) is even function, 

If f(x) is odd function, 

Even and Odd Functions - Examples

Example 1 : 

Let f(x)  =  x³, is f(x) odd or even function ?

Solution : 

To know f(x) is odd or even function, let us plug x = -x in f(x).

Then, we have 

f(-x)  =  (-x)³

f(-x)  =  -x³

f(-x)  =  - f(x)

So, f(x) is odd function.

Example 2 : 

Let f(x)  =  x²  + 2, is f(x) odd or even function ?

Solution : 

To know f(x) is odd or even function, substitute -x for x in f(x).

Then, we have 

f(-x)  =  (-x)² + 2

f(-x)  =  x² + 2

f(-x)  =  f(x)

So, f(x) is even function.

Example 3 : 

Let f(x)  =  x³ - 2x, is f(x) odd or even function ?

Solution : 

To know f(x) is odd or even function, substitute -x for x in f(x).

Then, we have 

f(-x)  =  (-x)³ - 2(-x)

f(-x)  =  -x³ +  2x

f(-x)  =  -(x³ - 2x)

f(-x)  =  -f(x)  

So, f(x) is odd function.

Example 4 : 

Let f(x)  =  5x³ + x² - 1, is f(x) odd or even function ?

Solution : 

To know f(x) is odd or even function, substitute -x for x in f(x).

Then, we have 

f(-x)  =  5(-x)³ + (-x)² - 1

f(-x)  =  5(-x³) - x² - 1

f(-x)  =  -5x³ - x² - 1

f(-x)  =  -(5x³ + x² + 1)

f(-x) can not be expressed as either as f(x) or -f(x).  

So, f(x) is neither even nor odd function.

Example 5 : 

Let f(x)  =  x⁴ + 2x²  + 5, is f(x) odd or even function ?

Solution : 

To know f(x) is odd or even function, substitute -x for x in f(x).

Then, we have 

f(-x)  =  (-x)⁴ + 2(-x)² + 2

f(-x)  =  x⁴ + 2x² + 2

f(-x)  =  f(x)

So, f(x) is even function.

Important Note : 

In trigonometric ratios, if we have negative angle, we have to understand that the angle will fall in the IV^th quadrant. 

In IV^th quadrant, the trigonometric ratios "cos" and "sec" are positive and all other trigonometric ratios are negative

Example 6 : 

Is sinx odd or even function ?

Solution : 

Let f(x)  = sinx

To know f(x) is odd or even function, substitute -x for x in f(x).

Then, we have 

f(-x)  =  sin(-x)

Because the angle is negative, it falls in the IV^th quadrant. In IV^th quadrant "sin" is negative.

So, we have

f(-x)  =  - sinx

f(-x)  =  - f(x)

 f(x) is odd function  

So, sinx is odd function.

Example 7 : 

Is cosx odd or even function ?

Solution : 

To know f(x) is odd or even function, substitute -x for x in f(x).

Then, we have 

f(-x)  =  cos(-x)

Because the angle is negative, it falls in the IV^th quadrant. In IV^th quadrant "cos" is positive.

So, we have

f(-x)  =  cosx

f(-x)  =  f(x)

f(x) is even function

So, cosx is even function.

Example 8 : 

Is tanx odd or even function ?

Solution : 

Let f(x)  = tanx

To know f(x) is odd or even function, substitute -x for x in f(x).

Then, we have 

f(-x)  =  tan(-x)

Because the angle is negative, it falls in the IV^th quadrant. In IV^th quadrant "tan" is negative.

So, we have

f(-x)  =  - tanx

f(-x)  =  - f(x)

 f(x) is odd function  

So, tanx is odd function.

Example 9 : 

Let f(x)  =  sinx + tanx, is f(x) odd or even function ?

Solution : 

To know f(x) is odd or even function, substitute -x for x in f(x).

Then, we have 

f(-x)  =  sin (-x) + tan(-x)

Because the angle is negative, it falls in the IV^th quadrant. In IV^th quadrant both "sin" and "tan" are negative.

So, we have

f(-x)  =  - sinx - tanx

f(-x)  =  - (sinx + tanx)

f(-x)  =  - f(x)

So, f(x) is odd function.

Note : The sum or difference of two odd functions is always odd. 

Example 10 : 

Let f(x)  =  secx + cosx, is f(x) odd or even function ?

Solution : 

To know f(x) is odd or even function, substitute -x for x in f(x).

Then, we have 

f(-x)  =  sec(-x) + cos(-x)

Because the angle is negative, it falls in the IV^th quadrant. In IV^th quadrant both "sec" and "cos" are positive.

So, we have

f(-x)  =  secx + cosx

f(-x)  =  f(x)

So, f(x) is even function.

Note : The sum or difference of two even functions is always even. 

After having gone through the stuff given above, we hope that the students would have understood even and odd functions. 

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