In the adjoining figure, ABCD is a quadrilateral in which AB||DC and AD||BC. Prove that 
=
.
Given AB||DC and AD||BC
Therefore, 
+
=180° _____________ (i)
+
=180° _____________ (ii)
From equations (i) and (ii)
+
= 
+
= 
Proved.
3
In the adjoining figure, ABCD is a quadrilateral in which AB||DC and AD||BC. Prove that =.
Given AB||DC and AD||BC
Therefore, +=180° _____________ (i)
+=180° _____________ (ii)
From equations (i) and (ii)
+ = +
= Proved.