When we formally prove the angle sum property of a triangle then we need to draw a line parallel to the base of triangle.. why this is so?any particular reason??

When we formally prove the angle sum property of a triangle then we ne

1.	When we formally prove the angle sum property of a triangle then we need to draw a line parallel to the base of triangle.. why this is so?any particular reason??
Answer» When we formally prove the angle sum property of a triangle then we need to draw a line parallel to the base of triangle.. why this is so?any particular reason??