Mid-point theorem


To verify the mid-point theorem for a triangle.

Theorem :

The line segment connecting the midpoints of  two sides of a triangle is parallel to the third side and is congruent to one half of the third side.

Basic concepts and facts

1.Parallel Lines:
Two lines are parallel if they do not meet at any point.
2.Congruent Triangles:
Two triangles are congruent if their corresponding angles and corresponding sides are equal.
3.Similar triangles:
Two triangles are similar if their corresponding angles equal and their corresponding sides are in proportion.


Proof of theorem:

Given in the figure A :


To prove:

PQ || BC and PQ=1/2 BC


To prove ▲ APQ ≅ ▲ QRC

Proof steps:

  1. AQ=QC [midpoint]
  2. ∠ APQ = ∠QRC [Corresponding angles for parallel lines cut by an transversal].
  3. ∠PBR=∠QRC=∠APQ [Corresponding angles for parallel lines cut by an transversal].
  4. ∠RQC=∠PAQ [When 2 pairs of corresponding angles are congruent in a triangle, the third pair is also congruent.]
  5. Therefore , ▲APQ ≅ ▲QRC
  6. AP=QR=PB and PQ=BR=RC.

Since midpoints are unique, and the lines connecting points are unique, the proposition is proven.