Practice "sometimes, always, never"
if AB lies in plane m and CD lies in plane n, and m is parallel to n, then AB is parallel to CD
sometimes
if PQ is the perpendicular bisector of RS, the point P lies on RS
sometimes
if each of two lines is skew to a third lines, then they are skew to each other
sometimes
if line k is perpendicular to plane m, and line j is parallel to m, then line k is skew to line j
sometimes
if two circles intersect at points X and Y then XY is the perpendicular of the segment joining the centers of the circles
sometimes
if two planes are parallel to the same line, they are parallel to each other
sometimes
in a space, two lines perpendicular to the same line are parallel
sometimes
planes that contain two skew lines are parallel
sometimes
theorems and postulates are reversible
sometimes
If four points in a plane are equidistant from the endpoints of a line segment in the same plane, then the points are collinear
always
definitions are reversible
always
if A, B, C and D are non coplanar, and AB is perpendicular to BC, and AB perpendicular to BD, then AB is perpendicular to the plane determines by B, C, D
always
if a line if oblique to a plane, it is perpendicular to exactly one line in a plane
always
if a line is perpendicular to two sides of a triangle, it is perpendicular to the median drawn to the third side
always
if two planes are perpendicular to the same lines, they are parallel to each other
always
if two points A and B are each equidistant from the endpoints of XY, then the midpoint of AB is also equidistant from the endpoints of XY
always
if two points of a line lie on a plane, then the entire lines lies on the plane
always
in a plane, two lines perpendicular to the same line are parallel
always
lines that never intersect are parallel
always
the sets of points in the space equidistant from the endpoints of a line segment forms a plane
always
three parallel lines are coplanar
always
two lines perpendicular to the same plane are parallel
always
two parallel lines determine a plane
always
Three lines, each perpendicular to the other two, all lie in the same plane
never
if a plane is oblique to a second plane, it contains lines perpendicular to the second plane
never
the perpendicular bisector of a side of a scalene triangle passes through the opposite vertex
never
two congruent complementary angles are right angles
never
If AB is the perpendicular bisector of CD, then CD is the perpendicular bisector of AB
sometimes
If two isosceles triangles have the same segment as a base, then the segment joining the vertices of the vertex angles of the triangles intersects the common base at its midpoints
sometimes
a line perpendicular to one of two perpendicular planes is parallel to the other
sometimes
