Theorem 10: The diagonals of a parallelogram bisect one another
On the diagram to the right, AC bisects BD and BD bisects AC . This means that:
|AE| = |EC| |BE| = |ED|
‘To bisect’ means to divide into two equal parts.
B
Proof of Theorem 10 The diagonals of a parallelogram bisect one another. Let ABCD be a parallelogram with AB ǁ CD and AD ǁ BC
1 B 2
Draw a diagonal line from A to C and a diagonal line from B to D. Let AC intersect BD at E.
B 3 |AD| = |BC| (opposite sides of a parallelogram)
AD ǁ BC Therefore, AC is a transversal and |∠EAD| = |∠ECB|
A (alternate angles)
Therefore, DB is a transversal and |∠EDA| = |∠EBC|
4 (alternate angles)
Look at triangle 1 ∆ ADE and triangle 2 ∆ BCE. From step 3:
|AD| = |BC|
|∠EAD| = |∠ECB| |∠EDA| = |∠EBC|
Therefore, triangle 1 ∆ ADE and triangle 2 ∆ BCE are identical (congruent) by angle-side-angle (ASA).
5
Since the triangles are congruent, |AE| = |EC| (corresponding sides are equal) AC bisected |BE| = |ED| (corresponding sides are equal) BD bisected The diagonals of a parallelogram bisect each other.
The full formal proofs are not examinable for Junior Cycle Mathematics, but you should be able to display understanding of the proofs.