A jet plane flying at a constant velocity v at a height=8km is tracked by a radar R located at O directly below the line of flight. If angle θ is decreasing at rate of 0.025rad/s, the velocity of plane when θ=60∘ is
A
1440 km/h
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B
960 km/h
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C
1920 km/h
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D
480 km/h
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Solution
The correct option is B
960 km/h tanθ=hx⇒xtanθ=h⇒cotθ=xh
Now, since v=dxdt ⇒v=h×cosec2θ×dθdt v=8×43×0.025×60×60km/h ⇒v=960km/h