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See diagram enclosed.
Take a triangle A B C.  Let AC be the base and B vertex on top. 
First fold the paper at A & C along AC. yu have a fold along AC. Discard piece of paper below AC. cut it off.
Fold The paper at B with fold parallel to AC. Fold must pass through B. Folder the paper inside, folded part goes behind the triangle.
Now open the fold. The angle between BC and the fold is angle C.  Fold the paper along AB. Press hard on the fold.Unfold now.
Angle between the new fold and the previous one is angle A.  The sum of angles at B now is : A + B + C = 180.  This is the angle on one side of the folded line along

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