Patrick B. answered 11/07/20
Math and computer tutor/teacher
OK here's how I drew it...
line v underneath u, and w underneath v...
so from north to south is u,v,w
stray line slices them
Let angle x be the vertical angle across from 1...
Let angle y be the vertical angle across from 2...
Let angle z be the vertical angle across from 3...
with respect to line u:
so on the "northeast corner of the junction is 1 and
then "southeast corner of the junction is x
with respect to line v:
so on the "northeast corner of the junction is 2 and
then "southeast corner of the junction is y
with respect to line w:
so on the "northeast corner of the junction is z and
then "southeast corner of the junction is 3
the proof follows from this chain of angle equalities,
which goes like this:
angle z = angle3 (vertical)
= angle1 (given)
= angle x (vertical)
= angle y ( corresponding via parallel u and v)
= angle 2 (vertical)
since all of these angles are equal, the three lines
must be parallel by converse theorem (lines cut by stray
such that corresponding angles are congruent forces parallel)