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Cubic Polynomial — Definition, Formula & Examples

Cubic Polynomial

A polynomial of degree 3. For example, x3 - 1, 4a3 - 100a2 + a - 6, and m2n + mn2 are all cubic polynomials.

Key Formula

f(x)=ax3+bx2+cx+df(x) = ax^3 + bx^2 + cx + d
Where:
  • aa = Leading coefficient (must not be zero)
  • bb = Coefficient of the squared term
  • cc = Coefficient of the linear term
  • dd = Constant term

Worked Example

Problem: Determine whether the polynomial 5x³ − 2x + 7 is cubic, and identify its degree, leading coefficient, and constant term.
Step 1: Find the highest power of x in the polynomial.
5x3−2x+7⇒highest power is 35x^3 - 2x + 7 \quad \Rightarrow \quad \text{highest power is } 3
Step 2: Since the highest power is 3, the degree is 3, which means this is a cubic polynomial.
Step 3: The leading coefficient is the number in front of x³, which is 5. The constant term is 7.
Answer: Yes, 5x³ − 2x + 7 is a cubic polynomial with degree 3, leading coefficient 5, and constant term 7.

Why It Matters

Cubic polynomials model many real-world situations, such as the volume of a box whose dimensions depend on a single variable, or the relationship between pressure and volume in certain physical systems. They can have up to three real roots and up to two turning points, giving their graphs a distinctive S-shaped or N-shaped curve. Understanding cubics also lays the groundwork for studying higher-degree polynomials.

Common Mistakes

Mistake: Forgetting that degree counts the sum of exponents in multivariable terms. For instance, assuming m²n is degree 2.
Correction: Add the exponents of all variables in a single term: m²n has degree 2 + 1 = 3, so it is indeed a cubic term.

Related Terms