Basic Mathematics & Logarithm
Inequalities
Grade 11

Question:

<p>If \(a < 0, b < 0, c < 0\) and \(a + b + c = abc\), then atleast one of the numbers \(a, b, c\) exceeds</p>
<p>(a) \(\dfrac{3}{2}\)</p>
<p>(b) \(\dfrac{17}{10}\)</p>
<p>(c) \(\sqrt{3}\)</p>
<p>(d) Other value</p>

Step-by-Step Solution

Key Concept: Use the constraint equation and AM-GM inequality on the absolute values to establish a lower bound on at least one variable.
<p><strong>Solution:</strong> Given \(a < 0, b < 0, c < 0\) and \(a + b + c = abc\)</p><p>Since all are negative, let \(a = -p, b = -q, c = -r\) where \(p, q, r > 0\).</p><p>Then \(-p - q - r = -pqr\), so \(p + q + r = pqr\)</p><p>By AM-GM inequality: \(\dfrac{p+q+r}{3} \geq \sqrt[3]{pqr}\)</p><p>This gives \(\dfrac{pqr}{3} \geq \sqrt[3]{pqr}\), which implies \(pqr \geq 3\sqrt{3}\)</p><p>At least one of \(p, q, r\) must exceed \(\dfrac{3}{2}\), hence at least one of \(a, b, c\) exceeds \(-\dfrac{3}{2}\) or is less than \(-\dfrac{3}{2}\).</p><p>∴ Answer is (a).</p>
Correct Answer: A

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