Let ABCD be a parallelogram, where AB ǁ CD and AD ǁ BC Draw a diagonal line from B to D .
Adiagonal is a line segment joining two vertices of a polygon, when those vertices are not on the same edge.
A 3 AD ǁ BC
Therefore BD is a transversal and |∠ABD| = |∠BDC| (alternate angles) |∠ADB| = |∠DBC| (alternate angles)
4 Look at triangle 1, ∆ ABD and triangle 2 ∆ BCD
The diagonal drawn in step 2 is a common side to both triangles |∠ABD| = |∠BDC| and |∠ADB| = |∠DBC| from step 3.
B 5
From step 4, we can say that the two triangles are congruent (identical) by angle-side-angle (ASA).
Therefore, in the parallelogram |AB| = |CD| (opposite sides are equal) |AD| = |BC| (opposite sides are equal) |∠DAB | = |∠BCD | (opposite angles are equal)
A B A 1 2 C D C D D
D
B
C
B
C
Converse 1 of Theorem 9: If the opposite angles of a quadrilateral are equal, then it is a parallelogram. Converse 2 to Theorem 9: If the opposite sides of a quadrilateral are equal, then it is a parallelogram.
Worked example
Find the values of A and B and the length of the sides marked x and y in each if the following diagrams. Justify each of you answers using an axiom or theorem or corollary used to fi nd the answer. (i)
x 14 cm A 12 cm 120° B 60° y x B 8 cm Solution (ii) 65° 21 m