Explanation
Solution:
1. Define the Variables:
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Let H be the height of the lamp post =10 m.
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Let h be the height of the man =2 m.
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Let x be the distance of the man from the lamp post.
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Let s be the length of the man's shadow.
2. Given Information:
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The man is moving away from the lamp post at a rate of 2 m/sec. Therefore, dtdx=2 m/sec.
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We need to find the rate of increase of the shadow length, which is dtds.
3. Set up the Equation:
Using the property of similar triangles from the geometry of the situation:
Total DistanceHeight of Lamp=Shadow LengthHeight of Man
4. Simplify the Relation:
Cross-multiply to solve for the relationship between x and s:
5. Differentiate with respect to time (t):
Substitute the given value dtdx=2 m/sec:
Correct Option:
(a) 21 m/sec