WOLFRAM|DEMONSTRATIONS PROJECT

A Line Crossing Two Parallel Lines Makes Alternate Angles Equal

​
angle
3.92699
Let the line
l
intersect the parallel lines
m
and
n
at
P
and
Q
. Then the alternate angles at
P
and
Q
are equal. To see this, rotate half of the figure by 180° around the midpoint of
PQ
to make the alternate angles coincide.