2d Shapes

Plane Geometry is the study of flat shapes that you see in pictures or on paper. These shapes include lines, circles, triangles, squares, and rectangles. We call them flat because they have length and width, but no thickness.

A side is a straight edge of a shape.

A vertex (more than one: vertices) is a point where two sides meet.

A polygon is a flat, closed shape made of straight sides that do not cross each other.

Look at the shapes below. Decide if each one is a polygon.\(\quad\)\(\quad\)\(\quad\)

A polygon is named by the number of sides it has. Here are some common polygons:

Triangle	Quadrilateral	Pentagon	Hexagon
3 sides	4 sides	5 sides	6 sides

A regular polygon has all sides the same length and all angles the same size.\(\quad\)\(\quad\)

A rectangle is a quadrilateral (a 4-sided polygon) with four right angles. In a rectangle, opposite sides are equal in length.

A circle is a flat shape. Every point on the circle is the same distance from a point called the center.
The radius is a line segment from the center to a point on the circle. The radius is also the length of this segment.
The diameter is a line segment that goes across the circle through the center and connects two points on the circle. The diameter is also the length of this segment.

• The diameter is twice as long as the radius: \(\textcolor{colorprop}{d}=2\times \textcolor{colordef}{r}\).
• The radius is half of the diameter: \(\textcolor{colordef}{r}=\textcolor{colorprop}{d} \div 2\).