In the diagram, PX is a tangent to the circle and RST is an equilateral…

Explanation

\(\bigtriangleup\) RST is equilateral triangle, hence

< TRS = < RTS = < RSt = 60^o

But < PTR = 60^o(Angle between a chord and a tangent at the point of contact = Angle in the alt. segment). From the diagram < PTS = < PTR + < RTS

= 60^o + 60^o = 120^o