# CBSE Class 10-Mathematics: Chapter –9 Some Applications of Trigonometry Part 11

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**Question 18:**

An aeroplane when flying at a height of form the ground passes vertically below another aeroplane at an instant when the angle of elevation of the two planes from the same point on the ground are respectively. Find the distance between the two aeroplane at that instant.

**Answer:**

Let be the positions of the two aeroplane

Let

And

Also

**Question 19:**

A boy standing on a horizontal plane finds a bird flying at a distance of 100 m from him at an elevation of . A girl standing on the roof of high building finds the angle of elevation of the same bird to be . Both the boy and the girl are on opposite side of the bird. Find the distance of the bird from the girl.

**Answer:**

Positions of bird at A, boy at P and girl at B are as shown in figure.

In

Also

In

Hence, the bird is away from the girl.

**Question 20:**

At a point on level ground, the angle of an elevation of a vertical tower is found to be such that its tangent is , on walking towards the tower, the tangent of the angle of elevation is . Find the height of tower.

**Answer:**

Suppose height of tower is h meter

In

In