Parallel Lines and Planes

Section 3 - 1

Definitions

Parallel Lines - coplanar lines that do not

intersect

Skew Lines - noncoplanar lines

that do not intersect

Parallel Planes - Parallel planes do not

intersect

THEOREM 3-1 If two parallel planes are cut by a third plane, then the lines of intersection

are parallel.

Transversal - is a line that intersects

each of two other coplanar lines in different points to produce interior

and exterior angles

ALTERNATE INTERIOR ANGLES -

two nonadjacent interior angles on opposite sides

of a transversal

Alternate Interior Angles

21

4

3

ALTERNATE EXTERIOR ANGLES - two nonadjacent exterior angles on opposite sides

of the transversal

Alternate Exterior Angles

65

87

Same-Side Interior Angles -

two interior angles on the same side of the

transversal

Same-Side Interior Angles

21

4

3

Corresponding Angles - two angles in

corresponding positions relative to two lines cut by

a transversal

Corresponding Angles

65

21

4

3

87

3 - 2

Properties of Parallel Lines

Postulate 10 If two parallel lines are cut

by a transversal, then corresponding angles are

congruent.

THEOREM 3-2 If two parallel lines are cut

by a transversal, then alternate interior angles

are congruent.

THEOREM 3-3 If two parallel lines are cut

by a transversal, then same-side interior angles

are supplementary.

THEOREM 3-4 If a transversal is

perpendicular to one of two parallel lines, then it is perpendicular to the other

one also.

Section 3 - 3

Proving Lines Parallel

Postulate 11 If two lines are cut by a

transversal and corresponding angles are

congruent, then the lines are parallel

THEOREM 3-5 If two lines are cut by a

transversal and alternate interior angles are

congruent, then the lines are parallel.

THEOREM 3-6 If two lines are cut by a

transversal and same-side interior angles are

supplementary, then the lines are parallel.

THEOREM 3-7 In a plane two lines

perpendicular to the same line are parallel.

THEOREM 3-8 Through a point outside a line, there is exactly one line parallel to the given

line.

THEOREM 3-9 Through a point outside a line, there is exactly one line perpendicular to the

given line.

THEOREM 3-10 Two lines parallel to a third line are parallel to

each other.

Ways to Prove Two Lines Parallel

1. Show that a pair of corresponding angles are congruent.

2. Show that a pair of alternate interior angles are congruent

3. Show that a pair of same-side interior angles are supplementary.

4. In a plane show that both lines are to a third line.

5. Show that both lines are to a third line

Section 3 - 4

Angles of a Triangle

Triangle – is a figure formed by the segments that join three noncollinear points

Scalene triangle – is a triangle with all three sides of different length.

Isosceles Triangle – is a triangle with at least two legs of equal length and a third side called the base

Angles at the base are called base angles and the third angle is the vertex angle

Equilateral triangle – is a triangle with three sides of equal length

Obtuse triangle – is a triangle with one obtuse angle (>90°)

Acute triangle – is a triangle with three acute angles (<90°)

Right triangle – is a triangle with one right angle (90°)

Equiangular triangle – is a triangle with three angles of equal measure.

Auxillary line – is a line (ray or segment) added to a diagram to help in a proof.

THEOREM 3-11 The sum of the measures of the angles of a triangle is 180

CorollaryA statement that can easily be proved by applying a theorem

Corollary 1 If two angles of one triangle are congruent to two angles of another triangle, then the third angles are congruent.

Corollary 2 Each angle of an equiangular triangle has measure 60°.

Corollary 3 In a triangle, there can be at most one right angle or obtuse angle.

Corollary 4 The acute angles of a right triangle are complementary.

THEOREM 3-12 The measure of an exterior angle of a triangle equals the sum of the measures of the two remote interior angles.

Section 3 - 5

Angles of a Polygon

Polygon – is a closed plane figure that is formed by joining three or more coplanar segments at their endpoints, and

Each segment of the polygon is called a side, and the point where two sides meet is called a vertex, and

The angles determined by the sides are called interior angles.

Convex polygon - is a polygon such that no line containing a side of the polygon contains a point in the interior of the polygon.

Diagonal - a segment of a polygon that joins two nonconsecutive vertices.

THEOREM 3-13 The sum of the measures of the angles of a convex polygon with n sides is (n-2)180°

THEOREM 3-14The sum of the measures of the exterior angles of a convex polygon, one angle at each vertex, is 360°

Regular Polygon

A polygon that is both equiangular and equilateral.

To find the measure of each interior angle of a regular polygon

3 - 6

Inductive Reasoning

Inductive ReasoningConclusion based on several past observations

Conclusion is probably true, but not necessarily true.

Deductive ReasoningConclusion based on accepted statements

Conclusion must be true if hypotheses are true.

THE END