Problem 94 - Precalculus : Building Concepts and Connections

Question

Is it possible for a quadratic function with real coefficients to have one real zero and one nonreal zero? Explain. (Hint: Examine the quadratic formula.)

Step-by-Step Solution

Answer

No, it is not possible for a quadratic function with real coefficients to have one real zero and one nonreal zero because complex roots come in conjugate pairs.

Step 1: Understanding Complex Roots

Recall that in any quadratic equation with real coefficients \( ax^2 + bx + c = 0 \), if the discriminant \( b^2 - 4ac \) is negative, then the roots of the quadratic equation are complex and they always appear in conjugate pairs. This means that if one root is \( p + qi \), the other root would be \( p - qi \). Here, \( p \) and \( q \) are real numbers, and \( i \) is the imaginary unit such that \( i^2 = -1 \).

Step 2: Conclude

Since complex roots always come in pairs for quadratic equations with real coefficients, it's not possible for a quadratic function with real coefficients to have only one real root and one complex root. In other words, if a quadratic equation with real coefficients has one complex root, it must also have the other (conjugate pair) complex root, which doesn't allow for a real root.

Key Concepts

Complex Roots

Understanding complex roots is crucial when dealing with quadratic equations. In quadratic functions with real coefficients, such as the standard form \( ax^2 + bx + c = 0 \), the presence of complex roots is determined by the discriminant. When the equation's roots are complex, they arise from a negative discriminant \( b^2 - 4ac < 0 \). This ensures that the solutions are complex numbers instead of real numbers.

Complex roots always appear in conjugate pairs in equations with real coefficients. A conjugate pair consists of two solutions: if one root is expressed as \( p + qi \), the second root automatically becomes \( p - qi \). In this pair, \( p \) and \( q \) are real numbers, while \( i \) is the imaginary unit, satisfying \( i^2 = -1 \). This pairing excludes the possibility of having a mix of one real and one nonreal (complex) root for any quadratic equation with real coefficients.

Quadratic Formula

The quadratic formula is an essential tool for finding the roots of any quadratic equation of the form \( ax^2 + bx + c = 0 \). The roots are given by the formula: \[ x = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}.\]

The "plus-minus" symbol (\( \pm \)) indicates that this formula is designed to provide both potential solutions for the equation. The term under the square root, known as the discriminant \( b^2 - 4ac \), determines the nature of the roots.

If the discriminant is positive, the quadratic has two distinct real roots. If it is zero, there is exactly one real root or a double root. If negative, as discussed in the section on complex roots, the equation yields two complex roots. This automatic calculation of the roots' nature is what makes the quadratic formula so powerful in solving quadratic problems.

Discriminant

The discriminant is a fundamental part of the quadratic formula and greatly affects the type of solutions that a quadratic equation has. It is represented by \( b^2 - 4ac \), derived from the coefficients of the equation \( ax^2 + bx + c = 0 \).

The discriminant shows:

• A positive value results in two real and distinct roots.
• A zero value gives one repeated real root, sometimes referred to as a double root.
• A negative value results in two complex conjugate roots.

Understanding how the discriminant affects the roots is essential when analyzing quadratic equations. It allows us to quickly assess the potential nature of the roots without solving the equation completely.