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HINTS:
PARALLEL LINES
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.
Proposition 1.
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
converse.

See also:
Problem 71

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