Applications of Derivatives
Angle between curve and axis
Grade 12

Question:

<p><strong>Ex. 24(C):</strong> If the curve <span class="math">y = 2e^{2x}</span> intersects the Y-axis at an angle <span class="math">\cot^{-1}\left|\frac{8n-4}{3}\right|</span>, then the value of <span class="math">n</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: Find the slope of the curve at the Y-axis intersection point and use the angle formula to match with the given expression.
<p><strong>Solution:</strong></p><p>The curve <span class="math">y = 2e^{2x}</span> intersects the Y-axis at <span class="math">(0, 2)</span>.</p><p>Slope: <span class="math">\frac{dy}{dx} = 4e^{2x}</span></p><p>At <span class="math">(0, 2)</span>: <span class="math">\frac{dy}{dx}\bigg|_{(0,2)} = 4</span></p><p>Angle of intersection with Y-axis: <span class="math">\theta = \tan^{-1}(4) = \cot^{-1}\left(\frac{1}{4}\right)</span></p><p>Given: <span class="math">\cot^{-1}\left|\frac{8n-4}{3}\right| = \cot^{-1}\left(\frac{1}{4}\right)</span></p><p>Therefore: <span class="math">\left|\frac{8n-4}{3}\right| = \frac{1}{4}</span></p><p><span class="math">8n - 4 = \pm\frac{3}{4}</span></p><p><span class="math">8n = 4 \pm \frac{3}{4} \Rightarrow n = 2 \text{ or } n = -1</span></p><p>∴ Answer: (r, q)</p>
Correct Answer: r, q

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