Dear student ,
In triangle PRS ,
RA = AB = BS -(i)
AB = CD = AB = BC -(ii)
From equation (i) and (ii) ,
AB = BC = CD = AD = RA = BS
DC || RS ,DC = 1/2 RS
Angle DCS + Angle RSC = 180 degree
Angle DCS = 180 degree
Angle RSC = 180 degree - 90 degree
Angle RSC = 90 degree
Hope it will help you
Regards ,
