Is defined by a line divides two sides of a triangle proportionally then it is parallel to the third?

Yes, if a line divides two sides of a triangle proportionally, it is indeed parallel to the third side. This is a result of the Basic Proportionality Theorem, also known as Thales' theorem. Essentially, the segments created on the two sides are in the same ratio as the lengths of the third side, confirming the parallelism. Thus, the line maintains the proportional relationships within the triangle.