LESSON NINETEENTH.
POLYGONS.
Name any thing besides your desk that has a flat surface.
A flat surface is called a plane.
How many sides has the plane Fig. A? (Diagram [16].)
It is called a triangle. “Tri” means “three.”
What other triangles do you see.
Triangles are sometimes called trigons.
A triangle is a plane figure having three sides.
How many sides has the plane figure marked B? How many angles?
It is called a quadrangle, or quadrilateral. “Quad” denotes “four.”
What other quadrangles do you see?
Why is Fig. B a quadrangle?
A quadrangle is a plane figure having four sides.
How many sides has the Fig. C?
It is called a pentagon.
What other pentagon do you see?
Why is Fig. C a pentagon?
A pentagon is a plane figure having five sides.
In like manner,—
A hexagon is a plane figure having six sides.
A heptagon is a plane figure having seven sides.
An octagon has eight sides.
A nonagon has nine sides.
A decagon has ten sides.
All these figures are called polygons.
“Poly” means “many.”
What do you call a polygon of three sides? Of four sides? Of six sides? &c.
If the length of each side of triangle A is one inch, how long are the three sides together?
The sum of the sides of a polygon is its perimeter.
Which of the triangles has unequal sides? Which has equal sides?
The latter is called a regular polygon.
Which pentagon has one side longer than any one of its other sides?
Which has its sides all equal to each other? Are its angles also equal?
It is therefore a regular polygon, or regular pentagon.
Name a hexagon that is not regular.
Name a regular hexagon.
A regular octagon. A regular heptagon.
A polygon is a plane figure bounded by straight lines.
