How to prove the sum of triangle

1.	How to prove the sum of triangle
Answer» TheoremIf ABC is a triangle then <)ABC + <)BCA + <)CAB = 180 degrees.ProofDraw line a through points A and B. Draw line b through point C and parallel to line a.triangleSince lines a and b are parallel, <)BAC = <)B\'CA and <)ABC = <)BCA\'.It is obvious that <)B\'CA + <)ACB + <)BCA\' = 180 degrees.Thus <)ABC + <)BCA + <)CAB = 180 degrees.

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