Two lines
l and m are perpendicular if they intersect
to
form right angle.
Line-Angle Relationship
If we are given two coplanar lines ( they lie in the same plane
) and the third
line which intersects both ( called transversal ), we can find:
a) 4 pairs of corresponding angles: 
1
and 5; 2
and 6; 4
and 8; 3
and 7
b) 2 pairs of alternate-interior angles ( AIP ):4
and 6; 3
and 5
c) 2 pairs of alternate-exterior angles ( AEP ): 1
and 7; 2
and 8
d) 2 pairs of same side interior angles ( SSIP ): 4
and 5; 3
and 6
If two lines are parallel ( line a II b ), then:
a) corresponding angles are equal in measure
b) alternate-interior angles are equal
c) alternate-exterior angles are equal
d) same side interior angles are supplementary
Examples: 1= 5
as corresponding angles; 4= 6
as AIP; 2= 8
as AEP; 3 + 6
= 180º as SSIP