📐 College Algebra & Precalculus

Quadratic Equation Calculator

Solve any second-degree equation of the form ax² + bx + c = 0. Get complete step-by-step discriminant derivations, real or complex roots, vertex coordinates, and an interactive plotted parabola.

Quick Presets:

Enter Coefficients

Must not be zero (a ≠ 0)

Current Equation:
x² - 5x + 6 = 0
Roots (Solutions)
x₁ = 3, x₂ = 2
Two distinct real roots
Parabola Vertex (h, k)
(2.5, -0.25)
Minimum point (Opens Upward)

📊 Interactive Parabola Curve

f(x) = ax² + bx + c

📝 Step-by-Step Derivation Breakdown

What is a Quadratic Equation?

A quadratic equation is a second-order polynomial equation in a single variable x, where the highest exponent of the variable is two. The standard mathematical form is:

ax² + bx + c = 0   (where a ≠ 0)

Here, a is the quadratic coefficient, b is the linear coefficient, and c is the constant term. When plotted on a coordinate Cartesian plane, every quadratic equation produces a smooth U-shaped curve known as a parabola.

The Quadratic Formula & The Discriminant

The universal algebraic formula used to find the roots of any quadratic equation is derived by completing the square on the standard equation:

x = (-b ± √(b² - 4ac)) / (2a)

The expression underneath the square root, Δ = b² - 4ac, is called the discriminant. It reveals the nature of the solutions without having to complete the entire formula:

  • If Δ > 0: The equation has two distinct real roots. The parabola crosses the x-axis at two distinct points.
  • If Δ = 0: The equation has exactly one real repeated root (double root). The vertex of the parabola touches the x-axis at a single tangent point.
  • If Δ < 0: The equation has two complex conjugate roots involving the imaginary unit i = √(-1). The parabola does not intersect the x-axis.

Step-by-Step Practice Examples

Example 1: Two Distinct Real Roots (x² - 5x + 6 = 0)

Identify coefficients: a = 1, b = -5, c = 6.

  1. Calculate discriminant: Δ = (-5)² - 4(1)(6) = 25 - 24 = 1.
  2. Since Δ = 1 > 0, there are two distinct real roots.
  3. Substitute into formula: x = (-(-5) ± √1) / (2(1)) = (5 ± 1) / 2.
  4. Root 1: x₁ = (5 + 1) / 2 = 6 / 2 = 3.
  5. Root 2: x₂ = (5 - 1) / 2 = 4 / 2 = 2.

Example 2: Complex Conjugate Roots (x² + 4x + 13 = 0)

Identify coefficients: a = 1, b = 4, c = 13.

  1. Calculate discriminant: Δ = 4² - 4(1)(13) = 16 - 52 = -36.
  2. Since Δ < 0, the roots are complex conjugate pairs: √(-36) = 6i.
  3. Substitute into formula: x = (-4 ± 6i) / 2.
  4. Solutions: x₁ = -2 + 3i,   x₂ = -2 - 3i.

Frequently Asked Questions

Can the coefficient 'a' be equal to zero?

No. If a = 0, the x² term disappears, leaving bx + c = 0, which is a linear equation rather than a quadratic equation. The quadratic formula requires division by 2a, which would lead to an undefined division by zero if a = 0.

What is the vertex of a parabola?

The vertex is the extreme turning point of the parabola. If a > 0, the parabola opens upward and the vertex represents the absolute minimum value. If a < 0, the parabola opens downward and the vertex represents the absolute maximum value. The x-coordinate of the vertex is always h = -b / (2a).

How does factoring relate to the quadratic roots?

If a quadratic equation has real roots r₁ and r₂, it can be factored into a(x - r₁)(x - r₂) = 0. By the Zero Product Property, setting either factor to zero yields the solutions x = r₁ or x = r₂.

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