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Finding x&y intercepts

A rational function is a function of the form R x p x q x , where p and q are polynomial functions and q 0 .

The domain is all real numbers except for numbers that make the denominator = 0.

x-intercepts are the points at which the graph crosses the x-axis. They are also known as roots, zeros, or solutions.

To find x-intercepts, let y (or f(x)) = 0 and solve for x. In rational functions, this means that you are multiplying by 0 so to find the x-intercept, just set the numerator (the top of the fraction) equal to 0 and solve for x.

Remember: x-intercepts are points that look like (x,0)

For y x 1 x 2 find the x-intercept

The x-intercept is (1,0) since x 1 0 , x 1

The y-intercept is the point where the graph crosses the y-axis. If the graph is a function, there is only one y-intercept (and it only has ONE name)

To find the y-intercept (this is easier than the x-intercept), let x = 0. Plug in 0 for x in the equation and simplify.

Remember: y-intercepts are points that look like (0,y)

For y x 1 x 2 find the y-intercept

The y-intercept is (0, -1 2 ) since 0 1 0 2 -1 2

Find the x- and y-intercepts of the following:

y 1 x 2

x-intercept: None since 1 0

y-intercept: (0, 1 2 ) since 1 0 2 1 2

y 1 3 x 1 x

x-intercept: ( 1 3 ,0) since 1 3 x 0 , -3 x -1 , x 1 3

y-intercept: (0,1) since 1 3 0 1 0 1

y x 2 x 2 9

x-intercept: (0,0) since x 2 0 , x 0

y-intercept: (0,0) since 0 2 0 2 9 0 9 0 or because the x-intercept is (0,0)

y x 1 x 2 2

x-intercept: (-1,0) since x 1 0 , x 1 0 , x 1

y-intercept: (0, 1 4 ) since 0 1 0 2 2 1 2 2 1 4

y 3 x x 2 x 2

x-intercept: (0,0) since 3 x 0 , x 0

y-intercept: (0,0) since the x-intercept is (0,0)

y 1 x 3 1

x-intercept: (2,0) since 1 x 3 1 0 , 1 x 3 1 , x 3 1 , x -2 , x 2

y-intercept: (0, 2 3 ) since 1 0 3 1 -1 3 1 2 3

y x 2 4 x 1

x-intercepts: (-2,0), (2,0) since x 2 4 0 , x 2 4 , x -2 , x 2

y-intercept: (0, -4) since 0 2 4 0 1 4 1 -4

y 4 5 x 2 2

x-intercept: None since 4 5 x 2 2 0 , 5 x 2 2 -4 , -4 x 2 8 5 , -4 x 2 13 , x 2 13 4 , a number squared will never be a negative number, so there is no x-intercept

y-intercept: (0, 13 2 ) since 4 5 0 2 2 4 5 2 13 2

y 5 x 2 x 3

x-intercept: ( 2 5 , 0) since 5 x 2 0 , 5 x 2 0 , 5 x 2 , x 2 5

y-intercept: None since y 5 0 2 0 3 takes the square root of a negative number.

y x 3 8 x 2 1

x-intercept: (2,0) since x 3 8 0 , x 2 x 2 2 x 4 0 , x 2

y-intercept: (0,-8) since 0 3 8 0 2 1 8 1 -8

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Source:  OpenStax, "rational"ity. OpenStax CNX. May 03, 2006 Download for free at http://cnx.org/content/col10350/1.2
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