от Xixibg » 20 Апр 2012, 15:15
[tex]\angle BAD=\angle BCD ; \angle ABC=\angle CDA[/tex]
Нека [tex]\angle ABC>\angle BAD ; =>\angle ABC-\angle BAD=70^\circ[/tex] (1)
[tex]\angle ABC+\angle BAD=180^\circ[/tex](2)
Събираме (1)и(2) и получаваме:[tex]\angle ABC-\cancel{\angle BAD}+\angle ABC+\cancel{\angle BAD}=180^\circ +70^\circ =250^\circ[/tex]
[tex]=>2\angle ABC=250^\circ ; =>\angle ABC=125^\circ[/tex]
[tex]\angle BAD=180^\circ-\angle ABC=180^\circ -125^\circ =55^\circ[/tex]