In the given figure, AB||CD. Prove that -=.
Draw a line EF||AB||CD.
+=180° [Because AB||EF and AE is the transversal] __________________ (i)
+=180° [Because DC||EF and CE is the transversal] __________________ (ii)
From equations (i) and (ii)
⇒ +=+
⇒ -=-
⇒ - = Proved.