Applications of Derivatives
Normal to curves
Grade 12

Question:

<p><strong>Ex. 24(D):</strong> If the area of a triangle formed by the normal at the point <span class="math">(1, 0)</span> on the curve <span class="math">x = e^{\sin y}</span> with the axes is <span class="math">\frac{|2t+1|}{6}</span> sq units, then the value of <span class="math">t</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 equation of the normal line at the given point, then calculate the area of the triangle formed with the coordinate axes.
<p><strong>Solution:</strong></p><p>Given: <span class="math">x = e^{\sin y}</span></p><p>Differentiating: <span class="math">\frac{dx}{dy} = \cos y \cdot e^{\sin y}</span></p><p>At point <span class="math">(1, 0)</span>: <span class="math">\frac{dx}{dy} = \cos(0) \cdot e^0 = 1</span></p><p>Therefore: <span class="math">\frac{dy}{dx} = 1</span></p><p>Slope of normal at <span class="math">(1, 0)</span>: <span class="math">m = -\frac{1}{1} = -1</span></p><p>Equation of normal: <span class="math">y - 0 = -1(x - 1) \Rightarrow x + y = 1</span></p><p>X-intercept: <span class="math">(1, 0)\,</span> Y-intercept: <span class="math">(0, 1)</span></p><p>Area of triangle with axes: <span class="math">\text{Area} = \frac{1}{2} \times 1 \times 1 = \frac{1}{2}</span></p><p>Given: <span class="math">\frac{|2t+1|}{6} = \frac{1}{2}</span></p><p><span class="math">|2t+1| = 3 \Rightarrow 2t + 1 = \pm 3</span></p><p><span class="math">t = 1 \text{ or } t = -2</span></p><p>∴ Answer: (p, s)</p>
Correct Answer: p, s

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