Hon. Geometry Fr Chris Thiel, OFMCap 2016 Parallel Lines Hon. Geometry Fr Chris Thiel, OFMCap 2016
Definitions Parallel-- coplanar lines that never intersect Skew--lines that never intersect because they are not coplanar Definitions
Transversal-- A line that goes across 2 or more lines
Angles Corresponding Angles Consecutive (Same Side) Interior Angles Same vs Alternate Side (of transversal) Angles Interior vs Exterior (of parallel) Corresponding Angles Consecutive (Same Side) Interior Angles Alternate Interior Angles Alternate Exterior Angles
Euclid’s 5th Postulate If consecutive (same side) interior angles are not supplementary, the lines are not parallel
Our Book’s approach Postulate: If and only if the lines are parallel, then Corresponding angles are congruent
Theorem: If and only if the lines are parallel, then Alternate Interior Angles are Congruent
Theorem: If and only if the lines are parallel, then Consecutive (Same Side) Interior Angles are Supplementary
Example: #20 p 88
Equivalent to Euclid’s Parallel Postulate (John Playfair’s axiom) Through a point outside a line, there is exactly one line parallel.
Theorem: The Sum of the Interior Angles of a Triangle is 180°
Remote Interior Angles Theorem
Regular Convex Polygons Not Regular Not Convex
Induction 2, 4, 6, 8, … 2, 3, 5, 7, 11, … 4, 5, 7, 10, 14, 19, … 1, 4, 9, 16, 25, 36, … 1, 8, 27, 64, 125, 216, …
Sum of Interior Angles of Convex Polygons:
Interior Angles of Regular Convex Polygons The supplement (the Exterior Angle) is... Sum of Exterior Angles of Convex Polygons
The Exterior angle of a Regular Pentagon The interior angle of a regular 36-gon If the interior angle is 188,° how many sides? The sum of interior angles of a 22-gon? The sum of exterior angles of a 173-gon?