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 .