Quadratic Equations
Extraneous roots / radical equations
Grade 11
Question:
<p>For the equation \(\sqrt{3x^2 + x + 5} = x - 3\), squaring both sides gives \(2x^2 + 7x - 4 = 0\), so \(x = \frac{1}{2}\) or \(x = -4\). Checking both roots in the original equation:</p>
<p>Both roots satisfy the equation</p>
<p>Only \(x = \frac{1}{2}\) satisfies the equation</p>
<p>Only \(x = -4\) satisfies the equation</p>
<p>No solution exists</p>
Step-by-Step Solution
Key Concept: Squaring both sides of radical equations introduces extraneous solutions; you must verify all solutions in the original equation, not just the squared version. A solution is valid only if it satisfies both the original equation AND the domain requirement (RHS ≥ 0).
**Step 1:** Square both sides of the equation.
$$ \sqrt{3x^2 + x + 5} = x - 3 $$
$$ 3x^2 + x + 5 = (x - 3)^2 $$
**Step 2:** Expand and simplify the equation.
$$ 3x^2 + x + 5 = x^2 - 6x + 9 $$
$$ 2x^2 + 7x - 4 = 0 $$
**Step 3:** Solve the quadratic equation for $x$.
$$ (2x - 1)(x + 4) = 0 $$
This yields two potential solutions: $x = \frac{1}{2}$ or $x = -4$.
**Step 4:** Check the potential solution $x = \frac{1}{2}$ in the original equation.
For $x = \frac{1}{2}$:
The Left Hand Side (LHS) is:
$$ \sqrt{3\left(\frac{1}{2}\right)^2 + \frac{1}{2} + 5} = \sqrt{3\left(\frac{1}{4}\right) + \frac{1}{2} + 5} = \sqrt{\frac{3}{4} + \frac{2}{4} + \frac{20}{4}} = \sqrt{\frac{25}{4}} = \frac{5}{2} $$
The Right Hand Side (RHS) is:
$$ \frac{1}{2} - 3 = \frac{1}{2} - \frac{6}{2} = -\frac{5}{2} $$
Since the LHS $\left(\frac{5}{2}\right)$ does not equal the RHS $\left(-\frac{5}{2}\right)$, $x = \frac{1}{2}$ is an extraneous solution. Furthermore, the RHS of the original equation, $x-3$, must be non-negative for a real square root to equal it. For $x=\frac{1}{2}$, $x-3 = -\frac{5}{2} < 0$, confirming it is extraneous.
**Step 5:** Check the potential solution $x = -4$ in the original equation.
For $x = -4$:
The LHS is:
$$ \sqrt{3(-4)^2 + (-4) + 5} = \sqrt{3(16) - 4 + 5} = \sqrt{48 - 4 + 5} = \sqrt{49} = 7 $$
The RHS is:
$$ -4 - 3 = -7 $$
Since the LHS $(7)$ does not equal the RHS $(-7)$, $x = -4$ is an extraneous solution.
**Conclusion:**
Both potential solutions, $x = \frac{1}{2}$ and $x = -4$, are extraneous. Therefore, the original equation has no real solution.
Correct Answer: D