Up | Japanese

Parallelism Conditions

---

• If a straight line intersects two straight lines, then each of the followings is necessary and sufficient for the two lines to be parallel:
  1. two alternate angles are equal in measure
  2. two corresponding angles are equal in measure
  3. two interior angles on the same side of a transversal are supplementary
  
  
• The sum of the inner angles of a triangle is 2 right-angle.
  
  
  This is proved by using the parallelism condition.
  
  
• The line segment joining the mid-points of two sides of a triangle is parallel to the third side and its length equals half of it.