166 Part 3: The Shape of the World
Facts about Triangles
A polygon is a figure formed by connecting line segments at their endpoints until they come back
to where they started. You need at least three segments to be able to close the shape, but you can
make a polygon with many more sides. The line segments are called sides, and the points where
the sides meet are called vertices. When two sides meet at a vertex, they form an angle.DEFINITION
A polygon is a closed figure made up of line segments that meet at their endpoints.The members of the family of polygons take more specific names based on the number of sides
they have. A polygon with three sides is called a triangle. It has three sides, three vertices, and
three angles. The prefix tri means “three.” Tri-angle = three angles. A polygon with four sides is
called a quadrilateral. Quad means “four” and lateral means “side.” We’ll look at quadrilaterals in
the next chapter.
Math is always interested in measuring things, and geometry is no exception. In a triangle, you
can measure the sides and the angles. You can also draw and measure other line segments inside
the triangle.
An altitude is a line segment that starts at a vertex of the triangle and meets the opposite side
at a right angle. You measure the altitude to find the height of the triangle, so often the words
altitude and height are used interchangeably.DEFINITION
An altitude is a line segment from a vertex of a triangle perpendicular to the opposite
side.The altitude usually falls inside the triangle,
but sometimes it can actually be a side of the
triangle, and sometimes, if the triangle has
an obtuse angle, the altitude can be outside
the triangle. If that happens, you extend the
opposite side to see where the altitude should
stop.This triangle has an obtuse angle, so the altitude falls
outside the triangle.BaseExtensionAltitude