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Take a triangle ABC, and have a line parallel to BC passing through A. Now we have line DAE II BC. Therefore, Angle B = angle DAB (alternate angles) Angle C = angle EAC (alternate angles) DAE is a line, therefore angle DAB + A + EAC = 180 degrees But angle B = DAB and C = EAC Hence proven, Angle A + B + C = 180 degrees.

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