Given a circle (1), the lines AED (2)
and BFC (3). If AB is parallel to CD, prove that the angles AFB
and AEB are equal.
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Proposition. If two lines
are parallel, each pair of alternate interior angles are
congruent. Also converse.
ANGLES IN A CIRCLE
Proposition. An inscribed angle is
measured by one-half its intercepted arc.
CYCLIC QUADRILATERAL is a quadrilateral whose
vertices all lie on a single circle.
Opposite angles of a cyclic (inscribed) quadrilateral are
supplementary. Also converse.
Proposition 2. A quadrilateral is cyclic if one side
subtends congruent angles at the two opposite vertices. Also