$\"\"$

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A plane moves horizontally at an altitude of $\"\"$ km.

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When the plane passes the telescope on the ground, the angle of elevation is $\"\"$ .

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$\"\"$

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Assume that plane be at the point $\"\"$ .

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Assume the telescope be at the point $\"\"$ .

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Assume that the horizontal distance traveled by the plane as $\"\"$ .

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Let the angle of elevation at any time $\"\"$ is $\"\"$ .

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Elevation angle is decreasing at a rate of $\"\"$ rad/min.

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Therefore, $\"\"$ .

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Consider $\"\"$ .

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$\"\"$

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Differentiate on each side with respect to $\"\"$ .

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$\"\"$

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Find the speed of the plane at $\"\"$ .

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Substitute $\"\"$ and $\"\"$ in $\"\"$ .

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$\"\"$

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Here the negative sign indicates that the distance is decreasing.

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Speed of the plane when the angle of elevation is $\"\"$ is $\"\"$ km/min.

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$\"\"$

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Speed of the plane when the angle of elevation is $\"\"$ is $\"\"$ km/min.