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Answer»

If 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.Since 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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