First we need to know what are vertical angles and a linear pair
vertical angles: The angles opposite each other when two lines cross. They are always equal, they share the same vertex.
linear pair: Two angles form a linear pair if they are supplementary (180°) and adjacent.
12. <1 and <2 linear pair
13. <1 and <3 vertical angles
14. <1 and <4 linear pair
15. <1 and <5 neither
16. <1 and <6 neither
17. <1 and <7 neither
18. <1 and <8 neither
19. <2 and <4 vertical angles
