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IB DP Maths AA SL Study Notes

2.3.2 Solving Quadratic Equations

Introduction to Quadratic Equations

At its core, a quadratic equation is a polynomial equation of the second degree. The solutions to these equations, often referred to as the roots or x-intercepts, are the values of x for which the equation holds true. Depending on the coefficients and the discriminant, a quadratic equation can have two distinct real roots, one repeated root, or no real roots at all. The methods to find these roots range from factoring to the quadratic formula, each with its unique advantages. For more on this, see our quadratic functions page.

Quadratic Formula: The Universal Method

The quadratic formula is a powerful tool that can solve any quadratic equation. It's derived from the process of completing the square and is given by:

x = (-b ± sqrt(b2 - 4ac)) / 2a

Example:

Consider the equation 2x2 - 4x - 6 = 0.

Using the quadratic formula, we can determine the roots: x1 = (4 + sqrt(40)) / 4 x2 = (4 - sqrt(40)) / 4

This method is especially useful when the equation isn't easily factorable. Understanding the domain and range basics can also be helpful when solving such equations. Visit our domain and range basics page for more details.

Factoring: The Elegant Solution

Factoring is one of the most intuitive methods for solving quadratic equations. It involves expressing the equation as a product of two binomials. This method is particularly effective when the equation can be easily factored.

Example:

For the equation x2 - 5x + 6 = 0, it can be factored as: (x - 2)(x - 3) = 0

This immediately gives two solutions: x = 2 and x = 3.

Completing the Square: Bridging the Gap

Completing the square is a method that rewrites the quadratic equation into a perfect square trinomial form. This method is foundational, as it's the basis for deriving the quadratic formula.

Example:

For the equation x2 - 4x - 5 = 0:

1. Add and subtract 4 to the equation: x2 - 4x + 4 - 5 - 4 = 0

2. Rewrite as a binomial square: (x - 2)2 = 9

3. Solve for x: x = 5 or x = -1

This method is similar to some techniques used in solving exponential equations.

Discriminant: Predicting the Nature of Roots

The discriminant, given by the expression b2 - 4ac, is a valuable tool in determining the nature of the roots without solving the equation.

  • If the discriminant is positive, there are two distinct real roots.
  • If it's zero, there's one repeated real root.
  • If it's negative, there are no real roots.

Example:

For the equation 3x2 - 6x + 3 = 0, the discriminant is: b2 - 4ac = 36 - 36 = 0

This indicates a repeated real root.

Real-world Applications of Quadratic Equations

Beyond the classroom, quadratic equations have vast applications:

1. Physics: They can model the motion of objects, especially in kinematics.

2. Business: Quadratics can represent profit functions, helping businesses optimise their operations. For instance, understanding the normal distribution can be crucial in certain business models.

3. Biology: They can model certain biological processes or phenomena.

Example:

A company finds that its profit, P, from selling x items is modelled by P = -2x2 + 50x - 120. To maximise profit, they need to determine the number of items to sell. Using the vertex formula, the optimal number of items is 12.5, but since they can't sell half an item, selling 12 or 13 items would yield the maximum profit. This principle is similar to finding the equation of a tangent line in calculus.

FAQ

Yes, the factoring method is effective for solving quadratic equations that can be easily expressed as a product of binomials. However, not all quadratic equations can be easily factorised. For equations that have irrational or complex roots, or those that don't factorise nicely over integers, the factoring method might not be the most efficient or feasible approach. In such cases, other methods like the quadratic formula or completing the square might be more appropriate.

Completing the square is a method that transforms a quadratic equation into a perfect square trinomial form. This method is foundational because it not only provides a direct way to solve quadratic equations but also serves as the basis for deriving the quadratic formula. By completing the square, students can gain a deeper understanding of the structure of quadratic equations and how their solutions relate to their graphical representations. It's a versatile technique that offers both algebraic and geometric insights into quadratics.

In real-world applications, the context often dictates the most suitable method for solving a quadratic equation. For instance, in physics problems where the motion of an object is modelled by a quadratic equation, factoring might be the preferred method if the equation is easily factorisable, as it can quickly provide interpretable results. However, in business or economics scenarios where precise values are needed, the quadratic formula might be more appropriate. The choice of method often hinges on the nature of the equation and the specific requirements of the application in question.

The quadratic formula provides the solutions or roots of a quadratic equation, which correspond to the x-intercepts of its graph. The x-intercepts are the points where the graph of the quadratic function touches or crosses the x-axis. By using the quadratic formula, students can determine these points and gain insights into the shape and position of the parabola. Additionally, the discriminant (part of the quadratic formula) can indicate whether the graph touches the x-axis (one repeated root), crosses it at two points (two distinct roots), or doesn't touch it at all (no real roots).

The discriminant of a quadratic equation, given by the expression b2 - 4ac, plays a crucial role in determining the nature of the roots of the equation without actually solving it. If the discriminant is positive, the quadratic equation has two distinct real roots. If it's zero, the equation has one repeated real root. If the discriminant is negative, the equation has no real roots, but two complex conjugate roots. Understanding the discriminant allows students to quickly assess the type of solutions they should expect and can also provide insights into the graph of the quadratic function.

Practice Questions

Solve the quadratic equation x^2 - 2x + 1 = 0.

To solve the equation x2 - 2x + 1 = 0, we can factorise it as (x - 1)(x - 1) = 0. This gives us a repeated root, x = 1. Therefore, the solution to the equation is x = 1.

A ball is thrown upwards from the ground with an initial velocity of 20 m/s. The height h of the ball after t seconds is given by the equation h = -5t^2 + 20t. Determine the time t when the ball reaches its maximum height.

To find the time t when the ball reaches its maximum height, we need to find the vertex of the parabola represented by the equation h = -5t2 + 20t. The x-coordinate (or in this case, t) of the vertex is given by the formula t = -b/2a. Using the coefficients from our equation, a = -5 and b = 20, we get t = -20/(-10) = 2. Therefore, the ball reaches its maximum height after 2 seconds.

Dr Rahil Sachak-Patwa avatar
Written by: Dr Rahil Sachak-Patwa
LinkedIn
Oxford University - PhD Mathematics

Rahil spent ten years working as private tutor, teaching students for GCSEs, A-Levels, and university admissions. During his PhD he published papers on modelling infectious disease epidemics and was a tutor to undergraduate and masters students for mathematics courses.

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