Angles and Lines 
Definitions and Axioms on Angles About a Point
An angle is formed when two lines meet at a point.
Angle ABC has arms AB and AC. Its vertex is B.
Angles are named by their size; that is, the amount of rotation or the number of degrees between the initial arm and the terminal arm.
An acute angle is greater than 0° but less than 90°
angle ABC is acute.
A right angle is exactly 90°. Angle DEF is a right angle.
An obtuse angle is greater than 90° but less than 180°
ANGLE GHI is obtuse.
A straight angle is exactly 180°. ANGLE JKL is a straight angle or straight line.
.
A reflex angle is greater than 180° but less than 360°. ANGLE MNO is a reflex angle.
A full circle measures 360°. The line OA has rotated through a full circle about the point O.
The sum of the six angles is 360°.
Vertically Opposite Angle Theorem When straight lines intersect, the Vertically opposite angles are equal. 
In the diagram above, angles 1 and 4, 2 and 5, 3 and 6 are pairs of Vertically opposite angles. That is, Angle 1 = Angle 4, Angle 2 = Angle 5, and Angle 3 = Angle 6.
Hint: Vertically  look for a V  the angles are across from each other in a V.
Two angles that sum to 90° are complementary angles. Two angles that sum to 180° are supplementary angles. 
Angle DEG and ANGLE GEF are complementary angles.
Angle FEG and ANGLE GEC are supplementary angles.
 axioms and definitions  parallel lines  practice  solutions 
.
Parallel lines are straight lines in the same plane which do not meet when produced for any finite distance in either direction.
A transversal is a straight line which meets two or more other straight lines. 
Theorems on Parallel Lines and their Angles
When a transversal meets a pair of parallel lines, it creates three classes of angles.
1) CoInterior Angles: are between the parallel lines, same side of the transversal.
In the diagram, angles 3 and 6 , angles 4 and 5, are pairs of cointerior angles.
2) Alternate Angles: so called because they alternate side of transversal and side of // line.
if one angle is left of the transversal, above the parallel line, (angle 1)
the other is right of the transversal, below the other parallel line. (angle 7)
2 kinds: interior alternates and exterior alternates
Interior alternates are inside or between the parallel lines.
In the diagram, angles 3 and 5 are a pair of interior alternate angles.
Angles 4 and 6 are also interior alternate angles.
Exterior alternates are outside the parallel lines.
In the diagram, angles 1 and 7 are a pair of exterior alternate angles.
Angles 2 and 8 are also exterior alternate angles.
3) Corresponding Angles: are angles on the same side of the transversal and the same side of the parallel lines.
if one angle is left of the transversal, above the parallel line,(angle 1)
the other is left of the transversal, above the other parallel line.(angle 5)
in the diagram angles 2 and 6, 3 and 7, 1 and 5, 4 and 8 are pairs of corresponding angles.
.
Parallel Lines Theorem1 When a transversal meets parallel lines

Parallel Lines Theorem2 When a transversal meets straight lines

So, in our diagram:
Angle 3 = Angle 5 and Angle 4 = Angle 6, interior alternate angles.
Angle 1 = Angle 7 and Angle 2 = Angle 8, exterior alternate angles.
Angle 1 =Angle 5, Angle 2 = Angle 6, Angle 4 = Angle 8, Angle 3 = Angle 7, corresponding angles.
Angle 4 + Angle 5 = 180° and Angle 3 + Angle 6 = 180° pairs of cointerior angles.
EXAMPLE 1:
In the diagram, EF is a transversal cutting the parallel lines AB and CD,
forming the indicated angles. Use the vertically opposite angle theorem and
the parallel lines theorems to list the equal and supplementary angles.
Solution:
1) By the opposite angle theorem, Angle 1 = Angle 3, Angle 2 = Angle 4, Angle 5 = Angle 7
and Angle 6 = Angle 8.
2) By alternate angles, Angle 3 = Angle 5 and Angle 4 = Angle 6; Angle 1 = Angle 7
and Angle 2 = Angle 8
3) By corresponding angles, Angle 1 = Angle 5, Angle 2 = Angle 6, Angle 3 = Angle 7,
and Angle 4 = Angle 8.
4) By cointerior angles, Angle 3 + Angle 6 = 180° and Angle 4 + Angle 5 = 180° .
.
 axioms and definitions  parallel lines  practice  solutions 
.
1/
2/ Find the sum of the measures of angle a and angle b.
3/ Find the measure of all the angles at E in the diagram.
 axioms and definitions  parallel lines  practice  solutions 
1/
corresponding angles 1 = 6 = 38º 
complement of Angle 1 Angle 2 = 90º – 38º = 52º 
Angle 3 = 90º given  alternate angles Angle 4 = Angle 6 = 38º 
vertically opposite Angle 5 = Angle 2 = 52º 
Angle 6 is opposite to 38º Angle 6 = 38º 
KE is a straight line, Angle 7 = 180º – 38º = 142º 
Angle 8 is opposite to 7 Angle 8 = Angle 7 = 142º 
2/
3/
 axioms and definitions  parallel lines  practice  solutions 
( Plane Geometry MathRoom Index )
(all content © MathRoom Learning Service; 2004  ).