Meeting at right
(90°) anglesAngles a, b, c, and d all equal 90 degrees.Flip and Negate
If two lines are perpendicular,
then their slopes
are negative reciprocals;
i.e., if l 1 has a slope of
2 and l 2 is perpendicular
to l 1 , then l 2 must have a
slope of -.The little box at the intersection of the two lines is the symbol for a right angle. If the lines are not
perpendicular to each other, then none of the angles will be right angles. Don’t assume that an angle is a
right angle unless you are specifically told that it is a right angle, either in the problem or with the 90°
symbol.
- When two parallel lines are cut by a third line, all of the small angles are equal, all of the
big angles are equal, and the sum of any big angle and any small angle is 180 degrees.
Parallel lines are two lines that never intersect, and the rules about parallel lines are usually
taught in school with lots of big words. But we like to avoid big words whenever possible.
Simply put, when a line cuts through two parallel lines, two kinds of angles are created: big
angles and small angles. You can tell which angles are big and which are small just by looking at
them. All the big angles look equal, and they are. The same is true of the small angles. Lastly,
any big angle plus any small angle always equals 180 degrees. (ETS likes rules about angles that
add up to 180 or 360 degrees.)
Parallel Lines