How do you find the domain and range of a function in interval notation?

1 Answer
Nov 18, 2014

There are so many different kinds of functions, but domain and range are important parts of your study of functions.

Let me give you some examples of polynomial functions:
y = 3x + 1, y = #x^2+3x + 2#, and y = #x^3#. Do you notice that each one of those functions has powers of x that are Whole numbers? Stick with those, and you will have a polynomial.

All polynomials have a domain of "All Real Numbers". In interval notation, we write: #(-\infty,\infty)#. On the horizontal number line, that covers all numbers from left to right (your x-axis).

Polynomials with ODD degree (highest power of x) stretch their way from low to high through all real numbers in the vertical direction. This means that their Range is "All Real Numbers" again: #(-\infty,\infty)#. Once these functions get going in those directions, you will never see the end of them! We call this their "End behavior".

Polynomials with EVEN degree must have either a maximum or minimum value. If the graph has a minimum value, then its y-values (Range) stretch from that number, up to #\infty#. We write that Range as #[min, \infty)#. Look at the graph shown below, it has a minimum (vertex) at (2,-4). Its Range would be #[-4,\infty)#.My screenshot

Notice that whenever we use the #\infty# symbols, we use a round ( or ). That means that we can not include a numeric value for the infinities. When we use the square [ or ], it refers to an actual value that is included in the function.

Your study of domain and range has just begun, and will include a wide variety of functions besides polynomials. When you discover a new function that behaves differently, carefully analyze its input and output values. You are on the way to great things!