A Line Crossing Two Parallel Lines Makes Alternate Angles Equal
Initializing live version

Requires a Wolfram Notebook System
Interact on desktop, mobile and cloud with the free Wolfram Player or other Wolfram Language products.
Let the line intersect the parallel lines
and
at
and
. Then the alternate angles at
and
are equal. To see this, rotate half of the figure by 180° around the midpoint of
to make the alternate angles coincide.
Contributed by: Jon Perry (March 2011)
Open content licensed under CC BY-NC-SA