For the proof here kindly check the attachment.
We are given that
. Also, the transversal is shown. Let us take the first case, that of
and
. Please note that all other proofs will follow in a similar manner.
Let us begin, please have a nice look at the diagram. We will see that
and
are vertically opposite angles. We know that vertically opposite angles are congruent. Thus,
and
are congruent angles.
=
Now, we know that
and
are alternate interior angles. We also, know that alternate interior angles are equal too. Thus, we have:
= 
From the above arguments it is clear that:
=
=
.
Thus,
= 
We have proven the first instance. Please note that all other instances can be proved in a similar fashion.
For example, for
and
we can take
and
as vertically opposite angles thus making
=
. Now,
and
are alternate interior angles and thus
and
are equal. Thus, we have
and
.