The exterior angle theorem, m∠1 + m∠2 = ∠4 is proved.
From given figure,
In triangle ABC, the sum of all three angles is equal to 180 degree.
[tex]\angle1+\angle2+\angle3=180............(1)[/tex]
In given diagram angle 3 and 4 form linear pair.
[tex]\angle3+\angle4=180........(2)[/tex]
From equation 1 and 2
[tex]\angle1+\angle2+\angle3=\angle3+\angle4\\\\\angle1+\angle2=\angle4[/tex]
Hence proved.
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