ANSWER

Step-by-step 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,
