The horizontal distance of the boat from the lighthouse is 17.58 ft.
The height of the lighthouse beacon, h the line of sight of the boat to the top of the beacon and the horizontal distance of the boat from the lighthouse form a right-angled triangle with opposite side, the height of the lighthouse and adjacent side the horizontal distance of the boat from the lighthouse, D.
Since the angle of elevation of the beacon is 9°, we have using trigonometric ratios that
tan9° = h/D
So, D = htan9°
Since h = 111 ft,
D = 111tan9°
D = 111 × 0.1584
D = 17.58 ft
So, the horizontal distance of the boat from the lighthouse is 17.58 ft.
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