4 Parallel Planes and Lines
4 Parallel Planes and Lines
4 Parallel Planes and Lines
• Intercepted segments
….
Line parallel to a plane
• Theorem VIII. If a straight line is parallel to a given plane, it is parallel to the
intersection of any plane through it with the given plane.
Proof: ….
Corollary 1. if a line outside a plane is parallel to some line in the plane, then
the first line is parallel to the plane
Corollary 2. if a line is parallel to a plane, then through any point there is a
line in the plane parallel to the given line
Corollary 3. The intersections of a plane with two parallel planes are parallel
lines
Planes perpendicular to a line are parallel
• Theorem IX. If each of two planes is perpendicular to the same line, they
are parallel; and conversely, if one of two parallel planes is perpendicular to
a line, the other is also.
Proof: ….
• Theorem XI. Through a point not in a plane there is one and only one
plane parallel to this plane.
Proof: ….
Planes parallel to given lines
• Theorem XII. Through one of two skew lines there is one and only one
plane parallel to the other line.
Proof: ….
Corollary. Two angles in space whose sides are parallel each to each are
either equal or supplementary.
Parallel planes intercept proportional
segments
• Theorem XV. If two straight lines are cut by three parallel planes, the
intercepted segments on one line are proportional to the
corresponding segments on the other.
Proof: ….