Identify zeros and their multiplicities

Graphs behave differently at various x-intercepts. Sometimes, the graph will cross over the horizontal axis at an intercept. Other times, the graph will touch the horizontal axis and bounce off.

Suppose, for example, we graph the function

Figure 7. Identifying the behavior of the graph at an x-intercept by examining the multiplicity of the zero.

The x-intercept [latex]x=-3\\[/latex] is the solution of equation [latex]\left(x+3\right)=0\\[/latex]. The graph passes directly through the x-intercept at [latex]x=-3\\[/latex]. The factor is linear (has a degree of 1), so the behavior near the intercept is like that of a line—it passes directly through the intercept. We call this a single zero because the zero corresponds to a single factor of the function.

The x-intercept [latex]x=2\\[/latex] is the repeated solution of equation [latex]{\left(x - 2\right)}^{2}=0\\[/latex]. The graph touches the axis at the intercept and changes direction. The factor is quadratic (degree 2), so the behavior near the intercept is like that of a quadratic—it bounces off of the horizontal axis at the intercept.

The factor is repeated, that is, the factor [latex]\left(x - 2\right)\\[/latex] appears twice. The number of times a given factor appears in the factored form of the equation of a polynomial is called the multiplicity. The zero associated with this factor, [latex]x=2\\[/latex], has multiplicity 2 because the factor [latex]\left(x - 2\right)\\[/latex] occurs twice.

The x-intercept [latex]x=-1\\[/latex] is the repeated solution of factor [latex]{\left(x+1\right)}^{3}=0\\[/latex]. The graph passes through the axis at the intercept, but flattens out a bit first. This factor is cubic (degree 3), so the behavior near the intercept is like that of a cubic—with the same S-shape near the intercept as the toolkit function [latex]f\left(x\right)={x}^{3}\\[/latex]. We call this a triple zero, or a zero with multiplicity 3.

Figure 8

For higher even powers, such as 4, 6, and 8, the graph will still touch and bounce off of the horizontal axis but, for each increasing even power, the graph will appear flatter as it approaches and leaves the x-axis.

For higher odd powers, such as 5, 7, and 9, the graph will still cross through the horizontal axis, but for each increasing odd power, the graph will appear flatter as it approaches and leaves the x-axis.

Example 6: Identifying Zeros and Their Multiplicities

Use the graph of the function of degree 6 to identify the zeros of the function and their possible multiplicities.

Figure 9

Solution

The polynomial function is of degree n. The sum of the multiplicities must be n.

Starting from the left, the first zero occurs at [latex]x=-3\\[/latex]. The graph touches the x-axis, so the multiplicity of the zero must be even. The zero of –3 has multiplicity 2.

The next zero occurs at [latex]x=-1\\[/latex]. The graph looks almost linear at this point. This is a single zero of multiplicity 1.

The last zero occurs at [latex]x=4\\[/latex]. The graph crosses the x-axis, so the multiplicity of the zero must be odd. We know that the multiplicity is likely 3 and that the sum of the multiplicities is likely 6.

Try It 2

Use the graph of the function of degree 5 to identify the zeros of the function and their multiplicities.

Figure 10

Solution

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