Question
A man on the top of a tower, standing on the sea-shore,
finds that a boat coming towards him takes 10 minutes for the angle of depression to change from 30° to 60°. How soon the boat reach the sea-shore?Solution
height of tower = h 
tan 30° = h / (x+y) tan 60° = h/x h/ √3 = x 1/√3 = h / (x+y) h√3 = x + y h√3 = h/√3 + y 2h/ √3 = y y = 2x time taken to travel y distance = 10mins time taken to travel x distance ( half of y distance) = 5 mins
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