# Functions

In the previous lesson, you learned about Cartesian coordinate system, graph of equations and linear equations. These are prerequisites to learn about function in this lesson.

A function is a relation between two sets – set A and set B, where each element $x$ of set A is related to exactly one element $y$ of set B. Note the word “exactly” which is very important.

The elements $x$ is called domain of function (or inputs) and the elements $y$ is called the range of function( or outputs).

## Rules for Function

A function must follow certain rules, otherwise it is not a function.

1. Every element of set A must be assigned to an element of set B.
2. Each element of set A must match with an element in set B.
3. An element of set B can be output of two or more elements of set A.
4. An element of set A cannot be input of two or more elements of set A. (See rule 2).

### What are the different ways to represent functions ?

There are 3 ways to represent functions.

Numerically – you can represent a function as set of ordered pairs (x, y) where x is the domain of the function and y is the range of the function.

For example, you can represent above diagram as

Algebraically – you can represent a function using algebraic notations. A function is denoted as $f(x)$.

For example, you are given an expression $x + y = 1$.

The above expression can be changed to following form

or

Graphically – you can represent a function using graph. A reveal lot of information about function such a increasing function, decreasing function, odd or even and other characteristics.

To represent a function using graph find the points required to plot the graph of the function. This is done using algebraic expression of the graph.

For example, let’s draw the graph of $f(x) = 1 - x$.

 x f (x) points (x, (f (x)) -1 2 (-1, 2) 0 1 (0, 1) 1 0 (1, 0) 2 -1 (2, -1)

The graph of the function using above points is given below.

## Piecewise-Defined Functions

A domain of a function is all real numbers. Sometimes a function is piecewise-defined. It means there are more than one function for different values of domain.

You need to specify two or more equations to represent a function with specific domain values.

For example,

Points to Remember

• If x value falls under the domain, then the function f(x) is defined for x and if x does not falls under the domain of f(x) then it is undefined for x.
• When the domain of the function is not mentioned then the implied domain is set of all real numbers.
• The range of the function must be a possible value after inserting x into the function f (x). The range falls between $\neg \infty$ to $+\infty$
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