Applications of Derivatives
Monotonicity of Cubic Functions
Grade 12

Question:

<p>If \(f(x) = x^3 + bx^2 + cx + d\) and \(0 \leq b^2 \leq c\), then</p>

Step-by-Step Solution

Key Concept: Analyze the derivative of f(x) to understand its monotonicity properties. Use the constraint 0 ≤ b² ≤ c to determine conditions on critical points and establish what can be definitively concluded about f(x).
<p><strong>Step 1:</strong> Find the derivative: f'(x) = 3x² + 2bx + c</p><p><strong>Step 2:</strong> Analyze the discriminant of f'(x): Δ = 4b² - 12c = 4(b² - 3c)</p><p><strong>Step 3:</strong> Apply the constraint 0 ≤ b² ≤ c. This gives us: b² - 3c ≤ c - 3c = -2c ≤ 0 (assuming c ≥ 0)</p><p><strong>Step 4:</strong> Since Δ = 4(b² - 3c) ≤ 0, the derivative f'(x) = 3x² + 2bx + c has no real roots (or one repeated root when b² = 0 and c = 0).</p><p><strong>Step 5:</strong> Since the leading coefficient of f'(x) is positive (3 > 0) and Δ ≤ 0, we have f'(x) ≥ 0 for all x ∈ ℝ, with equality only possible at isolated points.</p><p><strong>Step 6:</strong> This means f(x) is monotonically increasing (non-decreasing) on ℝ. Therefore, f can have at most one real root.</p><p><strong>Step 7:</strong> However, without additional information about the value of d or other boundary conditions, we cannot determine the exact nature of roots of f(x). The problem statement appears incomplete as it doesn't ask a specific question.</p><p><strong>∴ Answer:</strong> Unknown</p>
Correct Answer: Unknown

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