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4 Parallel Planes and Lines

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PROPERTIES OF THE PLANE:

Parallel planes and lines


AN Cahyono
Parallel planes and lines
• Parallel planes
….

• Line parallel to a plane


….

• 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: ….

Corollary 1. Parallel line-segments included between parallel planes are


equal.
Corollary 2. If a line is perpendicular to one of two parallel planes, it is
perpendicular to the other also.
Corollary 3. Two planes each parallel to a third plane are parallel to each
other.
Parallel planes
• Theorem X. If a plane is parallel to each of two intersecting lines, it is
parallel to the plane of these lines.
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: ….

• Theorem XIII. Through a point outside of each of two non-parallel


lines there is one and only one plane parallel to both of these lines.
Proof: ….
Angles whose sides are parallel
• Theorem XIV. If two intersecting lines in one plane are parallel,
respectively, to two intersecting lines in another plane, then the two
planes are parallel, and then the corresponding angles formed by the
lines are equal.
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: ….

Corollary. Parallel planes which intercept equal segments on any


transversal line intercept equal segments on every transversal line.
Thank you

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