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Topic:  sum of three angles of a triangle 
Related Item:  http://mathforum.org/mathtools/tool/1510/ 
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Subject:  180 degree proof (triangle) 
Author:  Max Knauer 
Date:  Jan 7 2004 
parallel lines, with one of them intersected at a single point by two other
lines (not parallel). The two intersecting lines then form a triangle between
the two parallel lines. Then, after demonstrating to students that angles
opposite, sharing a vertex and formed by the same two lines are congruent, and
that "opposite interior" angles are also congruent, the three angles formed
"outside" of the parallel lines by the single point of intersection of the two
intersecting lines can be found congruent to the identical interior angles of
the triangle formed between the parallel lines.
 
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