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If two lines are perpendicular,
then they form congruent, adjacent angles

If two lines form congruent, adjacent angles,
then the lines are perpendicular

If the exterior of two acute, adjacent angles are perpendicular,
then the angles are complementary

If two angles are supplements of congruent angles (or of the same angle),
then the two angles are congruent

If two angles are complements of congruent angles (or of the same angle),
then the two angles are congruent

Parallel Lines
coplanar lines that do not intersect

Skew Lines
noncoplanar lines that are neither intersecting nor parallel

Parallel Planes
planes that do not intersect

A line and a plane are parallel...
if they do not intersect

If two parallel planes are cut by a third plane,
then the lines of intersection are parallel

Transversal
a line that intersects two or more coplanar lines in different points

AlternateInterior Angles
two nonadjacent interior angles on opposite sides of the transversal (always congruent)

SameSide Interior Angles
two interior angles on opposite sides of the transversal (always supplementary)

Corresponding Angles
two angles in corresponding positions relative to the two lines (always congruent)

If two parallel lines are cut by a transversal,
then corresponding angles are congruent

If two parallel lines are cut by a transversal,
then alternateinterior angles are congruent

If two parallel lines are cut by a transversal,
then same side interior angles are supplementary

If a transversal is perpendicular to one of two parallel lines,
then it is perpendicular to the other one also

If two lines are cut by a transversal and alternate interior angles are congruent,
then the lines are parallel

If two lines are cut by a transversal and sameside interior angles are supplementary,
then the lines are parallel

In a plane, two lines perpendicular to the same line...
are parallel

Through a point outside a line
there is exactly one line parallel to the given line

Through a point outside a line,
there is exactly one line perpendicular to the given line

Two lines parallel to a third line
are parallel to each other

