A person standing on the bank of a river observes that the angle of elevation of the top of a tree on the opposite bank is 60 degrees. When he moves 40 m away from the bank, the angle becomes 30 degrees. Find the width of the river.

Two poles of heights 6 m and 11 m stand on a plane ground. If the distance between their feet is 12 m, find the distance between their tops.

From a point on the ground, the angle of elevation of the top of a tower is alpha. On walking a distance d toward the tower, the angle becomes beta. The height of the tower is:

The angle of elevation of the top of a tower from two points at distances a and b from the base (on the same side) are complementary. The height of the tower is:

From the top of a 60 m tower, the angles of depression of the top and bottom of a building are 30 degrees and 60 degrees respectively. Find the height of the building.