If the discriminant equals 0, the quadratic has:
AInfinitely many roots
BTwo distinct real roots
CNo real roots
DTwo equal real roots
Answer & Solution
Correct answer: D. Two equal real roots
The nature of the roots of a quadratic equation $ax^2 + bx + c = 0$ is determined by the discriminant, denoted by $D$ or $\Delta$, where:
$$D = b^2 - 4ac$$
The roots of the quadratic equation are calculated using the quadratic formula:
$$x = \frac{-b \pm \sqrt{D}}{2a}$$
Depending on the value of $D$, three cases arise:
1. **If $D > 0$**: The square root $\sqrt{D}$ is a positive real number. This results in two distinct real roots: $x_1 = \frac{-b + \sqrt{D}}{2a}$ and $x_2 = \frac{-b - \sqrt{D}}{2a}$.
2. **If $D = 0$**: The square root $\sqrt{0}$ is $0$. The formula becomes:
$$x = \frac{-b \pm 0}{2a} \implies x = \frac{-b}{2a}$$
Both roots are exactly the same value, resulting in **two equal real roots** (also called a repeated root).
3. **If $D < 0$**: The square root of a negative number is not a real number, so there are **no real roots**.
Since the question specifies $D = 0$, the quadratic has two equal real roots.
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