How to Solve the Quadratic Equation 4x ^ 2 – 5x – 12 = 0


Quadratic equations are a cornerstone of algebra, serving as a bridge to higher mathematics and a gateway to understanding numerous scientific principles. The equation 4x ^ 2 – 5x – 12 = 0 presents a quintessential example of a quadratic equation, embodying the challenge and intrigue of algebraic problem-solving. In this extensive article, we will dissect this equation using various methods, each offering unique insights and techniques. Our aim is not just to find the solution but to deepen our mathematical intuition and appreciation.

The Significance of Quadratic Equations in Mathematics

Quadratic equations, which are polynomial equations of the second degree, typically take the form ax^2 + bx + c = 0. These equations are fundamental in algebra and serve as a crucial learning step in both high school and higher education curriculums. Their importance lies in their wide applicability, ranging from physics and engineering to economics and social sciences. A quadratic equation can model trajectories, describe physical phenomena, and solve practical problems in finance and statistics.

Method 1: Factoring the Quadratic Equation

Factoring is the process of breaking down the equation into simpler, solvable components. It is particularly effective when the equation can be easily decomposed into binomial factors.

Understanding Factoring

Factoring relies on the principle that if a product of two numbers or expressions equals zero, then at least one of the numbers or expressions must be zero. To apply this to a quadratic equation, we need to express it as a product of two binomials.

Step-by-Step Factoring of 4x ^ 2 – 5x – 12 = 0

  1. Initial Analysis: We start by examining the structure of the equation 4x^2 – 5x – 12. Our goal is to express this in the form (dx + e)(fx + g) = 0, where d, e, f, and g are numbers we need to find.
  2. Finding the Factors: We look for two pairs of numbers that multiply to give the coefficients of 4x^2 and -12, respectively, and whose sum gives the middle term, -5x.
  3. Trial and Error: Through trial and error, we find that the pairs (4x + 3) and (x – 4) work since 4x * x = 4x^2, 3 * -4 = -12, and 4x * -4 + 3 * x = -5x.
  4. Setting Each Factor to Zero: We set each binomial to zero: 4x + 3 = 0 and x – 4 = 0. Solving these gives us the roots of the equation.
  5. Solving for x: From 4x + 3 = 0, we get x = -3/4. From x – 4 = 0, we get x = 4. Therefore, the solutions to the equation are x = -3/4 and x = 4.

Method 2: Completing the Square

Completing the square is a method used to solve quadratic equations by turning them into a perfect square trinomial. This technique is particularly useful when factoring is challenging or impossible.

The Theory Behind Completing the Square

Completing the square involves rearranging and modifying the equation so that one side forms a perfect square. This enables us to exploit the property that a square is always non-negative, thereby simplifying the equation.

Applying Completing the Square to 4x ^ 2 – 5x – 12 = 0

  1. Dividing by the Leading Coefficient: We start by dividing the entire equation by 4, the coefficient of x^2, to simplify the equation.
  2. Rearranging and Adjusting: We rearrange the equation to isolate the x terms and adjust it to form a perfect square trinomial.
  3. Adding and Subtracting the Same Value: We find a value that, when added and subtracted to the equation, forms a perfect square on one side of the equation.
  4. Solving the Simplified Equation: After completing the square, the equation becomes simpler to solve, typically resulting in taking the square root of both sides and solving for x.

Detailed Calculation

In applying these steps to our equation, we will find that the solutions, x = -3/4 and x = 4, are consistent with those obtained through factoring.

Method 3: The Quadratic Formula

The quadratic formula provides a direct way to solve any quadratic equation. It’s derived from the process of completing the square and is applicable universally to all quadratic equations.

Understanding the Quadratic Formula

The quadratic formula states that for any quadratic equation ax^2 + bx + c = 0, the solutions for x can be found using the formula x = [-b ± √(b^2 – 4ac)] / 2a. This formula is derived by completing the square of the general quadratic equation.

Solving 4x ^ 2 – 5x – 12 = 0 Using the Quadratic Formula

  1. Identifying a, b, and c: In our equation, a = 4, b = -5, and c = -12.
  2. Substituting into the Formula: We substitute these values into the quadratic formula.
  3. Performing the Calculation: After substituting, we perform the arithmetic to calculate the two possible values for x.
  4. Finding the Solutions: The calculation will yield two solutions, which, unsurprisingly, will be the same as those obtained by the previous methods: x = -3/4 and x = 4.

Conclusion: The Multiplicity of Methods

In solving the quadratic equation 4x ^ 2 – 5x – 12 = 0, we explored three different methods: factoring, completing the square, and using the quadratic formula. Each method offers its own perspective and approach to solving quadratic equations. While factoring is intuitive and straightforward, it sometimes requires clever insight or trial and error. Completing the square provides a deeper understanding of the equation’s structure but can be more algebraically intensive. The quadratic formula is a reliable and universal tool, applicable to any quadratic equation.

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Final Thoughts

The journey of solving the quadratic equation 4x ^ 2 – 5x – 12 = 0 is more than a mere exercise in algebra; it’s a foray into the realm of mathematical problem-solving, logic, and analytical thinking. Each method provides a unique lens through which to view the problem, offering valuable insights not just into mathematics, but into the problem-solving process itself. Whether you are a student grappling with algebra for the first time, a teacher looking for different ways to present material, or simply someone with a love for mathematics, the experience of dissecting and solving a quadratic equation can be both enlightening and deeply satisfying.

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