Sometimes

If two angles are supplementary to the same angle, then they are supplementary to each other.

Never

Parallel lines are cut by a transversal and alternate interior angles are not congruent.

Always

Passing through a point not on line l, there exists exactly one line perpendicular to line l.

Always

Passing through point P not on line l, there exists exactly one line parallel to line l.

Sometimes

the converse of a theorem is true

Always

A theorem can be proven

Always

Two intersecting lines are coplanar

Sometimes

Supplementary angles form a linear pair

Sometimes

If line 1 is in plane X and line 2 is in plane Y and X is parallel to Y, then line 1 is parallel to line 2.

Always

If points M and N are on line PQ and line PM is congruent to line NQ, then line PN is congruent to line MQ

Always

An isosceles triangle __________ has exactly two congruent angles

Always

An equilateral triangle is _________ an equiangular triangle

Sometimes

A point that is equidistant from the endpoints of a segment _________ lies on a perpendicular bisector of that segment

Always

The measure of an exterior angle of a triangle is ________ equal to the sum of the measures of two interior angles

Sometimes

Alternate interior angles are ___________ congruent

Always

Vertical angles are __________ congruent by definition

Sometimes

Parallel segments are ________ congruent