4. In Fig. 6.31, if PQ  ST, 
 PQR = 110° and 
 RST = 130°, find 
 QRS.
[Hint: Draw a line parallel to ST through point R.]

                
                  Draw a line AB parallel to the ST through R.
Since PQ || ST and ST || AB 
 AB || PQ

PQR = 
QRB = 110°   (Alternate interior angles)
QRB = 
QRS + 
SRB .............(i)
Again, RST + 
SRB = 180°  (Interior angles of two parallels ST and RB)
=> SRB = 180° - 130° =  50°  ( 
RST = 130° )
Thus, QRS =