2(Rcos 30)........?????????

Since they are sitting at equal distance on boundary, the lines joining the positions form an equilateral triangle, and hence the length of string of each phone will be the length of the equilateral triangle's side.

Also the line from the centre of circle and through any vertex will bisect the angle 60^o/2.

so side of the equilateral triangle = 2(Rcos 30^o) = 2x20x3/2 = 203 m.

{

where, R is the radius of the circle. And in cos 30^o = Adjacent side/hypotenuse = adjacent side/R,

the triangle is OAB, O - centre of the circle, A - any vertex, OB perpendicular from the O onto side with one end A, and it bisetcts the side. OAB = 30^o

^}

This is also the length of telephone wire.

Regards,

Team,

TopperLearning.