Equations - Cubic

Cubic Equation

Definition and Characteristics

A cubic equation is a polynomial equation of degree 3. Its general form is: \[ ax^3 + bx^2 + cx + d = 0 \] where \( a \neq 0 \). The graph of a cubic function can have one or two turning points and may intersect the x-axis up to three times.

Graphical explanation of cubic function

Properties

  • Always has at least one real root.
  • Can have either one or three real roots.
  • Graph can have up to two turning points (local maxima and minima).
  • End behavior: As \( x \to \infty \), \( y \to \infty \) if \( a > 0 \); and \( y \to -\infty \) if \( a < 0 \).

Special Cases

  • Factorable form: \( (x - r_1)(x - r_2)(x - r_3) = 0 \)
  • Perfect cube identity: \( a^3 + b^3 = (a + b)(a^2 - ab + b^2) \)
  • Symmetric or inflection point at the origin in simpler forms like \( y = x^3 \)

Applications

  • Used in physics for volume and kinematics problems.
  • In engineering to model nonlinear relationships.
  • In economics to analyze cost, revenue, and profit functions with inflection points.
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