A vertical cliff 60 m high rises straight from the edge of a river in Meghalaya. From the top of the cliff, the angle of depression of a point at water level on the opposite bank, directly across, is 45°. A boat rows straight across from that point at a steady speed of 3 m per second. The width of the river and the time taken by the boat to cross are:

Question

Accepted Answer

60 m and 20 s. The angle of depression 45° equals the angle of elevation from the far bank, so tan 45 = 60/w => w = 60 m. Time = 60/3 = 20 seconds. Using 30° by mistake would give 60 √3 = 103.9 m.

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