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 =