EXAMPLE 5 A man walks along a straight path at a speed of 5 ft s A searchlight is located on the ground 12 ft from the path and

Question

EXAMPLE 5 A man walks along a straight path at a speed of 5 ft s A searchlight is located on the ground 12 ft from the path and is kept focused on the man At what rate is the searchlight rotating when the man is 5 ft from the point on the path closest to the searchlight SOLUTION We draw the figure to the left and let x be the distance from the man to the point on the path closest to the searchlight We let be the angle between the beam of the searchlight and the perpendicular to the path We are given that dx dt 5 ft s and are asked to find de dt when x 5 The equation that relates x and 0 can be written from the figure X tan 0 x tan 0 Differentiating each side with respect to t we get SO dx dt de dt de dt dx dt 5 When x 5 the length of the beam is 13 so cos 0 12 and 13 de dt The searchlight is rotating at a rate of rad s

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