How do you determine the domain and range of a graph inequalities?

1 Answer
Apr 1, 2015

To determine the domain is the same as to determine which numbers appear as the first number (the x-value) in an ordered pair that is part of the graph.
To determine the range is the same as to determine which numbers appear as the second number (the y-value) in an ordered pair that is part of the graph.

Here are some examples:

y>=x^2+3

graph{y >= x^2+3 [-11.6, 13.72, 0.15, 12.81]}

Although it is not 100% certain from just the graph, this graph does get wider and wider. Every x does appear in some ordered pair on the graph. The domain is all real numbers.

The y values that appear start at 3 and go up. We get all numbers greater than or equal to 3. Inequality: y >=3.
If you've learned interval notation, you write: [3, oo)

x^2 + y^2/9 <= 1

# graph{x^2 + y^2/9 <= 1 [-5.35, 7.14, -3.105, 3.14]}

Domain (x-values) Go from -1 to 1 (inequality: -1<= x <= 1)(interval: [-1, 1])

Range: -3 to 3 (inequal: -3 <= y <= 3(interval: [-3, 3])

More challenging:

x^2+y^2 < 9

graph{x^2+y^2 < 9 [-5.35, 7.14, -3.105, 3.14]}

The dotted line is not included, so we do not include the points at -3 or at 3.

Domain: -3<x<3 (interval (-3, 3)) Range: -3<y<3 (interval (-3, 3))

Last one:

x- y^2 < 6

graph{x- y^2 < 6 [-9.87, 30.68, -9.96, 10.32]}

Domain: all real numbers
Range: all real numbers