A man of height 2 meters walks at a uniform speed of 7 meters per minute away from a lamp post of height 9 meters. The rate (in meters per minute) at which the length of his shadows increases is

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answer is A.

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Detailed Solution

$\frac{dx}{dt} = 7 \frac{9}{2} = \frac{s + y}{y} \Rightarrow 9 y = 2 x + 2 y \Rightarrow 7 y = 2 x 7 \frac{dy}{dt} = 2 \frac{dx}{dt} \Rightarrow 7 \frac{dy}{dt} = 2 \times 7 \Rightarrow \frac{dy}{dt} = 2$

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