Question 1039195
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For line-by-line proof see the lesson <A HREF=https://www.algebra.com/algebra/homework/Triangles/Sum-of-the-interior-angles-of-a-triangle.lesson>Sum of the interior angles of a triangle</A> in this site.


It is not a two-column proof, although (!).


But I advise you to learn line-by-line proof first, before writing the two-column.

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Euclid wrote everything line-by-line.


Thousand years people learned and wrote Math line-by-line and knew everything.


In scientific math journals/magazines professionals write line-by-line only.
Nobody writes in two-columns.


In my generation people learned and wrote Math line-by-line and knew everything.


Now students are touched to write in two-columns and know nothing.


It is how contemporary USA math education system works now.