Two lines that are stretched into infinity and still never
intersect are called coplanar lines and are said to be parallel
lines. The symbol for "parallel to" is //.
If we have two lines (they don't have to be parallel) and have a
third line that crosses them as in the figure below - the crossing
line is called a transversal:
In the following figure:
If we draw to parallel lines and then draw a line transversal
through them we will get eight different angles.
The eight angles will together form four pairs of corresponding
angles. Angles F and B in the figure above constitutes one of the
pairs. Corresponding angles are congruent if the two lines are
parallel. All angles that have the same position with regards to
the parallel lines and the transversal are corresponding pairs.
Angles that are in the area between the parallel lines like
angle H and C above are called interior angles whereas the angles
that are on the outside of the two parallel lines like D and G are
called exterior angles.
Angles that are on the opposite sides of the transversal are
called alternate angles e.g. H and B.
Angles that share the same vertex and have a common ray, like
angles G and F or C and B in the figure above are called adjacent
angles. As in this case where the adjacent angles are formed by two
lines intersecting we will get two pairs of adjacent angles (G + F
and H + E) that are both supplementary.
Two angles that are opposite each other as D and B in the figure
above are called vertical angles. Vertical angles are always
Two lines are perpendicular if they intersect in a right angle.
The axes of a coordinate plane is an example of two perpendicular
In algebra 2 we have learnt how to find the slope of a line. Two
parallel lines have always the same slope and two lines are
perpendicular if the product of their slope is -1.
Video lesson: Find the value of x in the