A triangle is formed by three line segments which become sides of it. The lengths of the sides are exactly equal to the lengths of the respective line segments because the line segments are transformed as sides of the triangle.
Previously, the distance from one endpoint to another endpoint of a line segment is its length. Now, the distance from one interactive point of one endpoint to another interactive point of another endpoint of the line segment is length of that side. So, the distance from one vertex to another vertex in a triangle is length of the respective side.
Look at the triangle . The sides are , and and their lengths are , and respectively.