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Multiplicity

Multiplicity

How many times a particular number is a zero for a given polynomial. For example, in the polynomial function f(x) = (x – 3)4(x – 5)(x – 8)2, the zero 3 has multiplicity 4, 5 has multiplicity 1, and 8 has multiplicity 2. Although this polynomial has only three zeros, we say that it has seven zeros counting multiplicity.

 

 

See also

Double root, triple root, fundamental theorem of algebra

Key Formula

f(x)=a(x−r1)m1(x−r2)m2⋯(x−rn)mnf(x) = a(x - r_1)^{m_1}(x - r_2)^{m_2} \cdots (x - r_n)^{m_n}
Where:
  • r1,r2,…,rnr_1, r_2, \ldots, r_n = The distinct zeros (roots) of the polynomial
  • m1,m2,…,mnm_1, m_2, \ldots, m_n = The multiplicity of each corresponding zero
  • aa = The leading coefficient (a nonzero constant)

Worked Example

Problem: Find all zeros and their multiplicities for the polynomial f(x) = 2x⁴ − 12x³ + 18x², and describe how the graph behaves at each zero.
Step 1: Factor out the greatest common factor.
f(x)=2x2(x2−6x+9)f(x) = 2x^2(x^2 - 6x + 9)
Step 2: Factor the remaining quadratic. Recognize that x² − 6x + 9 is a perfect square trinomial.
f(x)=2x2(x−3)2f(x) = 2x^2(x - 3)^2
Step 3: Identify each zero and its multiplicity from the factored form. The factor x² gives zero x = 0 with multiplicity 2. The factor (x − 3)² gives zero x = 3 with multiplicity 2.
x=0 (multiplicity 2),x=3 (multiplicity 2)x = 0 \text{ (multiplicity 2)}, \quad x = 3 \text{ (multiplicity 2)}
Step 4: Check: the degree of the polynomial is 4, and the sum of the multiplicities is 2 + 2 = 4. This confirms we have found all zeros. Both zeros have even multiplicity, so the graph touches the x-axis at each zero and turns back — it does not cross.
2+2=4=deg⁡(f)2 + 2 = 4 = \deg(f)
Answer: The zeros are x = 0 with multiplicity 2 and x = 3 with multiplicity 2. At both zeros, the graph touches the x-axis and bounces back without crossing it.

Another Example

Problem: Determine the zeros and their multiplicities for g(x) = (x + 1)³(x − 4), and describe the graph's behavior at each zero.
Step 1: The polynomial is already in factored form. Read off each zero directly from the factors.
g(x)=(x+1)3(x−4)1g(x) = (x + 1)^3(x - 4)^1
Step 2: The factor (x + 1)³ gives the zero x = −1 with multiplicity 3. The factor (x − 4) gives the zero x = 4 with multiplicity 1.
x=−1 (multiplicity 3),x=4 (multiplicity 1)x = -1 \text{ (multiplicity 3)}, \quad x = 4 \text{ (multiplicity 1)}
Step 3: The zero x = −1 has odd multiplicity, so the graph crosses the x-axis there — but it flattens out as it crosses (an inflection-like crossing). The zero x = 4 has multiplicity 1, so the graph crosses the x-axis in a straight, non-flattening manner. The total multiplicities sum to 3 + 1 = 4, matching the degree.
Answer: x = −1 has multiplicity 3 (graph crosses with flattening), and x = 4 has multiplicity 1 (graph crosses without flattening).

Frequently Asked Questions

How does multiplicity affect the graph of a polynomial?
If a zero has odd multiplicity (1, 3, 5, …), the graph crosses the x-axis at that point. If a zero has even multiplicity (2, 4, 6, …), the graph touches the x-axis and turns around without crossing. Higher multiplicities produce a flatter appearance near the zero — the curve lingers closer to the axis before moving away.
What does it mean to count zeros 'with multiplicity'?
Counting with multiplicity means a repeated zero is counted as many times as it is repeated. A degree-5 polynomial always has exactly 5 zeros when counted with multiplicity (including complex zeros), even if some of those zeros are the same value. For instance, x = 2 with multiplicity 3 counts as three of those five zeros.

Simple zero (multiplicity 1) vs. Repeated zero (multiplicity ≥ 2)

A simple zero means the factor (x−r)(x - r) appears exactly once; the graph crosses the x-axis at a nonzero angle. A repeated zero means the factor appears two or more times. At a double root (multiplicity 2), the graph merely touches the axis. At a triple root (multiplicity 3), the graph crosses but with a distinctive flattening. In general, even multiplicity → bounce, odd multiplicity → cross.

Why It Matters

Multiplicity connects algebra and graphing in a powerful way: knowing a zero's multiplicity tells you immediately whether the graph crosses or bounces at that intercept. It also underpins the Fundamental Theorem of Algebra, which guarantees that a degree-nn polynomial has exactly nn zeros when counted with multiplicity (over the complex numbers). In calculus, a zero of multiplicity m≥2m ≥ 2 is also a zero of the derivative, which matters when analyzing critical points and curve behavior.

Common Mistakes

Mistake: Forgetting that the sum of all multiplicities must equal the degree of the polynomial.
Correction: Always verify by adding the multiplicities together. If their sum doesn't match the polynomial's degree, you have either missed a factor or miscounted a repeated one.
Mistake: Confusing the behavior at even vs. odd multiplicity zeros — thinking the graph always crosses at every x-intercept.
Correction: The graph only crosses at zeros with odd multiplicity. At zeros with even multiplicity, the graph touches the axis and turns back. Sketch a quick sign chart or test point if you're unsure.

Related Terms