A function is a concept of mathematics that studies the dependence between variable quantities in the process of their change. For instance, with a change in the side of a square, the area of the square also varies. The question of how the change in the side of the square affects the area is answered by a mathematical relationship between the area of the square and the side of the square.Let the variable x take numerical values from the set D.

A function is a rule that attributes to every number x from D one definite number y where y belongs to the set of Real numbers. Here, x is called the independent variable and y is called the dependent variable.

The set D is referred to as the **domain** of the definition of the function and the set of all values attained by the variable y is called the **range** of the function.

In other words, a variable y is said to be the value of the function of a variable x in the domain of definition D if to each value of x belonging to this domain there corresponds a definite value of the variable y.

This is symbolised as y = f(x) where f denotes the rule by which y varies with x.

## Even functions

A function is said to be even if for any value of x f(x) = f(-x)

**Properties**

- The sum, difference, product and quotient of even functions is also an even function.
- The graph of an even function is symmetrical about the y-axis.

Examples of even functions are y =x^{2}, y = x^{4}, y = |x|, cosΘ..... etc.

## Odd functions

A function is said to be odd if for any value of x f(x) = -f(-x)

**Properties**

- The sum and difference of odd functions are also an odd function.
- The product and quotient of odd functions are also an even function.
- The graph of an even function is symmetrical about the origin.

Examples of odd functions are y =x, y =x^{3}, y= x^{3} + x, y = sinΘ.... etc.

*Not all functions need to be even or odd. However, every function can be represented as the sum of an even function and an odd function.*

## Inverse of a function

Let there be a function y = f(x), which is defined for the domain D and has a range R. If the function y = f(x) is such that for every value of y (from the range of the function R) there corresponds one and only one value of x from the domain D, then the inverse function of y = f(x) exists and is given by x = g(y). Here it can be noticed that x becomes the dependent variable and y becomes the independent variable. Hence, this function has a domain R and a range D.

The graphs of two inverse functions when this change is used are symmetrical about the line y = x (which is the bisector of the first and the third quadrants).

## Plotting a function

A function y = f(x) can be represented in the coordinate plane where x values are plotted in the horizontal X axis and the corresponding result of f(x) are plotted on the y-axis. Assume a function y = f(x) gives the results in the y-row for values in the x-row in the given table.

X | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |

y=f(x) | 1 | 3 | 6 | 5 | 4 | 1 | 2 | 4 | 6 | 7 |

The image reflects the plot of the function.

## Shifting of graphs

The ability to visualize how graphs shift when the basic analytical expression is changed is a very important skill. The following points apply

- The shape of the graph y = f(x) + c will the same as that of the y = f(x) graph. The only difference would be that f(x) + c is shifted c units up(if c is positive) or down(if c is negative) on the x-y plot.
- The shape of the graph y = f(x ± c) will the same as that of the y = f(x) graph. The only difference would be that f(x ± c) is shifted c units left(if c is positive) or right (if c is negative) on the x-y plot.

## Plotting a quadratic curve

A function y = f(x) = ax^{2} + bx + c is a quadratic function. To plot this, we must convert the function into the form a(x+b)^{2} + c form. This can now be plotted using the shifting of graphs methods as discussed (f(x + a) + b shifting). The given diagram shows the graph of x^{2} + 4x +1 or (x+2)^{2} -3.