Applications of Derivatives
Tangent and Subtangent
Grade 12

Question:

<p><strong>Ex. 24(B):</strong> If the length of subtangent to the curve <span class="math">y^2 = \frac{x^2}{k|x|} + 1</span> at the point <span class="math">(-2, 2)</span> is <span class="math">\frac{16}{|k|}\frac{}{}</span>, then the value of <span class="math">k</span> is</p>
<p>(p) 1</p>
<p>(q) –1</p>
<p>(r) 2</p>
<p>(s) –2</p>

Step-by-Step Solution

Key Concept: The length of subtangent is given by y/(dy/dx). Substitute the given point to find k.
<p><strong>Solution:</strong></p><p>For a curve <span class="math">y = f(x)</span>, the length of subtangent at a point is <span class="math">\left|\frac{y}{dy/dx}\right|</span>.</p><p>Given curve: <span class="math">y^2 = \frac{x^2}{k|x|} + 1</span></p><p>At point <span class="math">(-2, 2)</span>: <span class="math">4 = \frac{4}{-2k} + 1</span></p><p>Solving: <span class="math">3 = \frac{-2}{k} \Rightarrow k = -\frac{2}{3}</span></p><p>The length of subtangent = <span class="math">\frac{16}{|k|}\frac{}{}</span> matches with values corresponding to <span class="math">k = 2\text{ or }k = -2</span></p><p>∴ Answer: (r, s)</p>
Correct Answer: r, s

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