CBSE Class 9 Answered

If the string of the telephone is such that it is streched to form a traingle.

Then the required triangle would be an equilateral triangle since three people are sitting at equal distance.

The centroid dividedwould be the centre of the circle.

We know that centroid divide the median in a ratio of 2:1

That means 2y =30

y=10

Now using pythagoras theorem

20²= (x/2)² + 10²

x = 20sqrt (3)

Alternate method:

All the angles of an equilateral traingle are 60⁰

That means central angle is 120⁰

In OBD

Using  trigonometric function

sin 60⁰= DB/ OB = (x/2)/20

sqrt(3)/2 = x/40

x = 20sqrt (3)