Lines and Angles Study Guide Introduction to Lines and Angles
There is geometry in the humming of the strings. —Pythagoras
(580–500 B.C.) Let's learn the lingo of lines and angles. Angles
can be measured with a tool called the protractor. A line is 180
degrees and can be thought of as a perfectly flat angle. A ray
is part of a line that has one endpoint. It extends indefinitely
in one direction. An acute angle is less than 90°. A right angle
equals 90°. A straight line is 180°. An obtuse angle is greater
that 90°. Two angles are supplementary if they add to 180°. Two
angles are complementary if they add to 90°. If you bisect an
angle, you cut it exactly in half. This forms congruent angles,
which have the same measure. When two lines intersect, four
angles are formed. The sum of these angles is 360 degrees. When
two lines are perpendicular to each other, their intersection
forms four 90-degree angles, which are also called right angles.
Right angles are identified by little boxes at the intersection
of the an