3.3 ' Geometrylines片and乌are parallel, denoted by£ 1 II£ 2. If two parallel lines are intersected by a third line, as
shown below, then the angle measures are related as indicated, where x + y = 18 0 °.y忱°
xo ＂
1 'y o
J;or X o
x0,( 乌
y^0- Polygons (Convex)
A polygon is a closed plane figure formed by three or more line segments, called the sides of the polygon.
Each side intersects exactly two other sides at their endpoints. The points of intersection of the sides are
vertices. The term "polygon''will be used to mean a convex polygon, that is, a polygon in which each
interior angle has a measure ofless than 180 °.The following figures are polygons:／二
The following figures are not polygons:zxo
A polygon with three sides is a triangle; with four sides, a quadrilateral; with five sides, a pentagon; and
with six sides, a hexagon.The sum of the interior angle measures of a triangle is 180°. In general, the sum of the interior angle
measures of a polygon with n sides is equal to (n - 2)180°. For example, this sum for a pentagon is
(5 - 2)180° = (3)180° = 540°.Note that a pentagon can be partitioned into three triangles and therefore the sum of the
angle measures can be found by adding the sum of the angle measures of three triangles.The perimeter of a polygon is the sum of the lengths of its sides.