A radar antenna is located on a ship that is 4km from a straight shore. It is rotating at 32rev/min. How fast does the radar beam sweep along the shore when the angle between the beam and the shortest distance to the shore is pi/4?

Let's say that the shoreline is y, are we looking for dy/dt?

r=radar beam
Using pythagarus, I found out that r=4root2

Would the equation be dy/dt=(dθ/dt)(dy/dθ)?

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  1. We don't really need r, because of the nice angle.

    y = 4tanØ
    dy/dt = 4sec^2 Ø (dØ/dt)

    so we need sec^2 (π/4) and dØ/dt
    but we are given dØ/dt = 32(2π) rpm , (one rotation is 2π radians)
    and cos π/4 = 1/√2
    then sec π/4 = √2
    and sec^2 π/4 = 2

    so dy/dt = 4(2)(32)(2π) km/min

    I will let you finish and convert to any unit you need.

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  2. Ohh, so that's how you find dy/dt, thanks!

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