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Exterior Angles of a transversal
Exterior Angles are created where a
transversal
crosses two (usually parallel) lines.
Each pair of these angles are outside the parallel lines, and on the same side of the transversal.
Try this Drag an orange dot at A or B. Notice that the two exterior angles shown are
supplementary (add to 180°) if the lines PQ and RS are parallel.
(If there is no image below, see support page.)
Referring to the figure above, the
transversal AB crosses the
two lines
PQ
and  RS
, creating intersections at E and F.
Each pair of exterior angles are outside the parallel lines and on the same side of the transversal.
There are thus two pairs of these angles. Click on the 'next' command to visit both pairs of exterior angles in turn.
Remember: exterior means outside the parallel lines. The parallel case
If the transversal
cuts across parallel lines (the usual case) then exterior angles are supplementary (add to 180°).
So in the figure above, as you move points A or B, the two angles shown always add to 180°.
Try it and convince yourself this is true. Click on the 'next' command to visit both pairs of exterior angles in turn.
The non-parallel case
If the transversal
cuts across lines that are not parallel, the exterior angles have no particular relationship to each other.
Drag point P or Q to make the lines non-parallel. As you move A or B, you will see that the exterior angles have no particular relationship to each other. Other parallel topicsGeneralAngles associated with parallel lines
(C) 2007 Copyright John Page
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