ANSWER
EXPLANATION
If ABCD is a parallelogram, then
line AB is parallel to line DC .
This means that,
and
are alternating angles.
Alternate angles are congruent.
This implies that,
We group like terms to obtain,
This simplifies to,
Also, if ABCD is a parallelogram then,
BC is parallel to AD. This means that,
We group like terms to get,
This simplifies to,
We divide both side by 4 to get,